captain_crunch

When good designers produce ugly boats.

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1 minute ago, Rasputin22 said:

Did you keep one on your dog tag chain?

I kept mine in the director.

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On 12/3/2018 at 1:48 AM, Fleetwood said:

Yeah, five sails up on a 30'(?) boat and it's still not going anywhere.....

You try moving 48,000 lbs (?) with 150 square feet of sail and see how fast you go.

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14 hours ago, bridhb said:

They were weird shaped fins and didn't take grounding very well.  People filled in the trailing edge but took a rating hit if it was known.  I had a 27 for about 6 years, a few boats ago.  My favorite boat that I have owned to date for the way it sailed, looked and "cruised".  They were not put together very well though but were easy to fix.  The 33T and 36T looked like big 27's.  Something bad happened when they were drawing the 30-2.  It had a very ugly "clipper" shape to the bow that the other "racers" didn't have.  Not sure why they did that.  The original 30  (and 24) CB boats were very nice looking and sailed well.  The 24 was a real sleeper around here a long time ago.

The Morgan 24 had a near magical balance that allowed her to sail very well to her MORC rating.   The 30 raced well too, but did not do quite as well in the 24.

My father had a Morgan 24 and one of his favorite stories was from one of the beer can/Thursday night races.   His crew did not make it, so he took her out anyway to race her single handed.   After a downwind start (back then, Thursday Night was non-spinnaker), he was holding his own in the class down to the first mark.   The next leg was a beat.

The boat was so well tuned and balanced that once the sails were trimmed in for upwind, she would sail, hands free, going upwind.   He was starting to make horizon on a boat that was two feet longer, so he hunkered down leeward, and then went below.   He was watching the other boat by peeking at the edge of the curtains as he starting to pass the other boat 20 feet to weather of them.   They were getting rather grumpy being passed by a smaller boat, until the cockpit came abeam of them and were dumbfounded to be passed by a sailboat with nobody on deck!

Dad had a lot of fun on that boat, and I was glad that I shared quite a bit of time with him.

- Stumbling

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14 minutes ago, stumblingthunder said:

The Morgan 24 had a near magical balance that allowed her to sail very well to her MORC rating.   The 30 raced well too, but did not do quite as well in the 24.

My father had a Morgan 24 and one of his favorite stories was from one of the beer can/Thursday night races.   His crew did not make it, so he took her out anyway to race her single handed.   After a downwind start (back then, Thursday Night was non-spinnaker), he was holding his own in the class down to the first mark.   The next leg was a beat.

The boat was so well tuned and balanced that once the sails were trimmed in for upwind, she would sail, hands free, going upwind.   He was starting to make horizon on a boat that was two feet longer, so he hunkered down leeward, and then went below.   He was watching the other boat by peeking at the edge of the curtains as he starting to pass the other boat 20 feet to weather of them.   They were getting rather grumpy being passed by a smaller boat, until the cockpit came abeam of them and were dumbfounded to be passed by a sailboat with nobody on deck!

Dad had a lot of fun on that boat, and I was glad that I shared quite a bit of time with him.

- Stumbling

Reminds me of the stories my grandfather told me about being a bombardier on a B-24 in WWII. When they were flying in the company of B-17s, they cut the two inboard engines between the them and the 17 and keep up just fine thanks their high aspect/low drag wings.

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Beer:

That's a potent looking catboat.

Many ears ago I was talking to Lowell North and I asked him what he though the most efficient rig was. He said, "One big sail." I thought of that when the wing sails came out on the AC boats.

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That catboat is bespoke everything. They built it at the Independence Seaport Museum in Philadelphia where the original is on display. 

Our A Cat fleet here is 28’ boat with a 48’ mast and a 36’ boom and the Silent Maid at 33’ dwarfs them all.

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I'd love to sail it. Not sure I'd love jibing it though. That must be fun. There was a 25' catboat that Bill Garden designed that lived in Port Townsend for many years. Fabulous looking boat. Huge 25'er. I love catboats, all that helm and all. Just part of the fun.

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3 hours ago, SloopJonB said:

The 36T was a One Ton, the 33T was a 3/4 Ton and the 30/2 was a MORC boat with a bit of a nod to the IOR.

As Bob said, the 27 was way big to be a 1/4 Tonner - might have made 1/2 Ton but with a 25' waterline it would have been tough to make the 21.7 rating of the time.

All very neat boats - they were some of the earliest to have those wedge decks.

Pretty sure the 27 was designed more to the MORC rule.  It had a very long waterline for a 27' boat from that era and a very fair hull with no weird bumps.  I raced it some in PHRF and even with very old sails it seemed to do pretty good,  sailed by 3 people, each with a beer in hand.  The keel was actually sort of "advanced".  Good foil shape and a substantial, almost bulb like thickened bottom.  If I didn't have stupid dreams of doing some real ocean  cruising in my old age (anyone want a 58 pound, 2 year old "chunky", really smart and friendly female (spayed) dog that showed up out of the woods a year ago?) I would consider another 27 or a 24.

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We called them P-38s. Never heard them called a P-51.

Every case. There were always more than enough.

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21 hours ago, Bob Perry said:

I think the problem with the BC tank was that it had become too small.

I did undergrad at the UBC tank (in Vancouver, not Victoria). My TA sailed on the first modern Canada 12m. The towing carriage computer was a PDP-11 with 8" floppy discs!

They tore it down because UBC could make way more $$$$ in selling the land for condo's. It was a pretty small tank too but it was useful.  And the big basin was great for storms at sea for the movie industry. Near the end they were making way more money booking movies.

Now we do CFD work in house at a fraction of the cost of tank time. We have 5 guys doing it and the CFD cluster has it's own 15 kW air conditioner and it's running 24/7. If you ever need an unusual hull analysed Bob, let me know.

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36 minutes ago, Zonker said:

I did undergrad at the UBC tank (in Vancouver, not Victoria). My TA sailed on the first modern Canada 12m. The towing carriage computer was a PDP-11 with 8" floppy discs!

They tore it down because UBC could make way more $$$$ in selling the land for condo's. It was a pretty small tank too but it was useful.  And the big basin was great for storms at sea for the movie industry. Near the end they were making way more money booking movies.

Now we do CFD work in house at a fraction of the cost of tank time. We have 5 guys doing it and the CFD cluster has it's own 15 kW air conditioner and it's running 24/7. If you ever need an unusual hull analysed Bob, let me know.

How well do CDF results match up with tank tests?

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Very good these days if we're talking resistance. Very useful to compare hull #1 & #2 and see which is better.

We can do a lot in CFD that would be cost-prohibitive in the tank. Studies like a low speed waterjet self-propelled test where we actually modelled the flow of water through the jets (and resulting resistance). Otherwise you'd  have to build teeny tiny waterjet ducts into a model with teenier impellers. This was a displacement hull (normally you only use waterjets on high speed boats where the transom is immersed at rest, and dry at speed). Dragging an immersed transom around at displacement speed with the flow interactions of the transom and the jets was interesting.

We also model tugs being dragged through the water sideways  (escorting work) when you're simulating trying to stop a tanker and you're getting flow interactions with the propellers which are spinning. The tug's skeg is acting like a wing and the hull is developing lift and it all gets very complicated.

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12 minutes ago, Zonker said:

Dragging an immersed transom around at displacement speed with the flow interactions of the transom and the jets was interesting.

Does reverse work at all with the jet reflecting against the transom?

I've seen quite a few extravagant claims from the marketing departments of high end 3d printers lately. Maybe one day soon tiny model bits will be cheap, but the models would still suffer from scaling issues I suppose.

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When I was at Michigan, there was an active towing tank (West Engineering, with one end near the arch) - I got to ride the carriage during student orientation.  

I'd often sit and eat lunch next to the shop so I could look at all the models.  Lotta tankers, but some very cool shapes. 

I think they use the building for philosophy or something now. 

 

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7 minutes ago, Kirwan said:

When I was at Michigan, there was an active towing tank (West Engineering, with one end near the arch) - I got to ride the carriage during student orientation.  

I'd often sit and eat lunch next to the shop so I could look at all the models.  Lotta tankers, but some very cool shapes. 

I think they use the building for philosophy or something now. 

 

The tank is still there and being used.

The NA department moved up to a building on North Campus the summer of 1977, but kept the tank in West Engineering - it would cost too much to build another one.

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In a tank you can't "scale" water. That is a big problem. That's why the towed model got bigger and bigger. Some people think that was where the Chance designed MARINER went wrong. The drag problems at the chopped off stern  were not visible in the tank. Maybe you could solve this by filling the tank with alcohol  or some dilution of it. But hell, then I could not smoke my pipe!  CFD eliminates this issue.

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1 minute ago, slap said:

The tank is still there and being used.

The NA department moved up to a building on North Campus the summer of 1977, but kept the tank in West Engineering - it would cost too much to build another one.

Good to hear - I guess I'd thought all of Engineering had moved, and assumed the tank was abandoned.

It's almost 'hidden', Lots of people would go through that building with no clue it's there. 

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On 12/3/2018 at 5:03 PM, Bob Perry said:

Whoaboy:

I have only done tank testing once and that was for my 42' double ended powerboat LION'S CONCERTO.

Tank testing is expensive. I think for 2.5 days of tank testing the cost was about $60,000. That included two 6' models. It does not include the client's time or my own time.

It was a lot of fun and very revealing. Most clients I get are not wiling to put $60,000+ into tank testing. Most of the shapes I draw are very conventional and I don't think I need any

tank work.  Don't get me wrong. I'd do it on every design if I had the budget. On the powerboat I was concerned about the double ended hull config so I suggested we tank test. We built two models at Janeki and took them to the now gone tank in Victoria BC. We were able to make small adjustments to the models and run them for two days getting immediate read outs of the performance after each run. My client loved it. I have a lot of photos of the process but none that are scanned. It's a shame they tore that tank down. Not enoug business to keep it running.

Don't let the haters bother you. They have nothing better to do and zero to contribute.

Thank you for the great answer, Mr Perry. 

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On 12/3/2018 at 4:54 PM, Bob Perry said:

I agree with Cruncher. The Vega is sort of the boat equivalent  of a Jeep. It has it's own look.

 

I've always like the look of the AV. Maybe I lean more towards the utilitarian ascetic.  

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21 hours ago, weightless said:

Does reverse work at all with the jet reflecting against the transom?

Yes, all waterjets are mounted on a vertical transom. To reverse, they lower "reverse buckets" into the flow - very much like the thrust reversers on a jet engine. The flow is diverted down a bit but mostly forward. Crash stop testing on a fast waterjet powered boats are great fun. Incredibly violent deceleration.

It's the bra shaped thing above the jet outlet. Whole thing swings down. The model shown is one of the larger ones Hamilton makes.

21610-12514869.jpg

 

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5 hours ago, woahboy said:

I've always like the look of the AV. Maybe I lean more towards the utilitarian ascetic.  

I always thought it looked like some sort of power boat adapted to sail - a Scandihoovian MacGregor trawler yacht if you will.

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1 hour ago, Zonker said:

Yes, all waterjets are mounted on a vertical transom. To reverse, they lower "reverse buckets" into the flow - very much like the thrust reversers on a jet engine. The flow is diverted down a bit but mostly forward. Crash stop testing on a fast waterjet powered boats are great fun. Incredibly violent deceleration.

It's the bra shaped thing above the jet outlet. Whole thing swings down. The model shown is one of the larger ones Hamilton makes.

21610-12514869.jpg

 

I think you win the internet, or at least our little corner of it, for the day with this photo. 

How long do those zincs typically last in service?

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It’s the modern derivative of sorts for the Kitchener rudder from England.

AC4C9967-8E5D-42DA-BABC-D4FC3DD1E92F.jpeg

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I suppose zincs last about a year or two. The waterjet is aluminum so if you install in a steel hull not as long as an aluminum or composite hull

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“modern derivative?”

Bollocks. (Unless you mean “modern” as being the jets dating from Bill’s advancement in the 1950s for internal turbine with direction thrust nozzle as being more modern than a 1914 patent and attempted an call of being “derivative” of a external shrouded prop?) The tech and phenomenon is very very different.

https://www.nzonscreen.com/title/jet-boats-in-new-zealand-1961

 

323F405C-615B-47FE-BFD4-50FCC4359465.jpeg

46EE7178-E081-495D-9AFF-9F6D0ADE1FCF.jpeg

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On 12/2/2018 at 3:10 PM, Great Red Shark said:

Yeah,  Froze - as well you may know, there's a couple of them that have been kicking around here (Oahu) nigh on the past 30 years - mostly doing daysail stuff and local cruising,  which can be pretty sporty. 

Oh, and somebody made the observation/crack that canoe sterns are some 'flawed concept' - now,  I'm not going to be arsed to find it, and no need to point fingers but...if someone WANTS that feature,  it's not a 'flaw' AND if you have some of the specific conditions that lend themselves to a double-ender,  you'll find yourself thinking they are pretty neat.

I have a bud with a big ol' Brewer double-ender and his current slip can build up an impressive stern sea in certain conditions - and his boat rides it like a magic carpet while every other boat on the row is beating an angry tatoo of slapping counters.   

I remember the one at KYC, my whole life growing up at KYC I think I can remember seeing that boat actually sail maybe a half dozen times. 

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On 12/4/2018 at 9:35 PM, Bob Perry said:

In a tank you can't "scale" water. That is a big problem. That's why the towed model got bigger and bigger. Some people think that was where the Chance designed MARINER went wrong. The drag problems at the chopped off stern  were not visible in the tank. Maybe you could solve this by filling the tank with alcohol  or some dilution of it. But hell, then I could not smoke my pipe!  CFD eliminates this issue.

Maybe a nice bourbon would be a better alternative. 

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Back when I was in college, I suggested to a physics professor that you could get all the magic ratios, e.g. Reynolds Number, more in line if the model was foreshortened. He dismissed  the idea out of hand.

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On 12/5/2018 at 10:27 PM, Zonker said:

Yes, all waterjets are mounted on a vertical transom. To reverse, they lower "reverse buckets" into the flow - very much like the thrust reversers on a jet engine. The flow is diverted down a bit but mostly forward. Crash stop testing on a fast waterjet powered boats are great fun. Incredibly violent deceleration.

It's the bra shaped thing above the jet outlet. Whole thing swings down. The model shown is one of the larger ones Hamilton makes.

21610-12514869.jpg

 

interesting piece of engineering!

 

Do you know if a jet is more or less fuel efficient than a traditional prop on a shaft?

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2 hours ago, Panoramix said:

interesting piece of engineering!

 

Do you know if a jet is more or less fuel efficient than a traditional prop on a shaft?

Yes.

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The crossover point is generally accepted to be around 25-30 knots. Above that range, the extra drag of a rotating prop shaft, P bracket, and rudder starts to really count. Below that range, the smaller impeller of a waterjet (and internal duct resistance) makes waterjets less efficient.

I also like surface drives for very fast boats. Really good efficiencies because you're turning a big prop and rudder chores are handled by swinging drive left/right (though some have external rudders). Not so good in reverse or at slow speeds where the waterjets still work OK

arneson-drive-on-manta.jpg

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5 minutes ago, Zonker said:

The crossover point is generally accepted to be around 25-30 knots. Above that range, the extra drag of a rotating prop shaft, P bracket, and rudder starts to really count. Below that range, the smaller impeller of a waterjet (and internal duct resistance) makes waterjets less efficient.

I also like surface drives for very fast boats. Really good efficiencies because you're turning a big prop and rudder chores are handled by swinging drive left/right (though some have external rudders). Not so good in reverse or at slow speeds where the waterjets still work OK

arneson-drive-on-manta.jpg

I'm trying to work up a kitchen setup to handle pulverizing lobster shells for the ultimate bisque. I may have the answer now.

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Just have to tilt the boat more vertically I would think. Otherwise the shells will spill out of the bowl.

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On 12/6/2018 at 9:34 AM, HFC Hunter said:

“modern derivative?”

Bollocks. (Unless you mean “modern” as being the jets dating from Bill’s advancement in the 1950s for internal turbine with direction thrust nozzle as being more modern than a 1914 patent and attempted an call of being “derivative” of a external shrouded prop?) The tech and phenomenon is very very different.

https://www.nzonscreen.com/title/jet-boats-in-new-zealand-1961

 

323F405C-615B-47FE-BFD4-50FCC4359465.jpeg

46EE7178-E081-495D-9AFF-9F6D0ADE1FCF.jpeg

I believe it was a tounge in cheek reference to an ancient technology.

Just that when you wanted reverse or to slow down the idea of forcing a reverse of thrust was similar in the most basic concept possible

 

And don’t get me started about the addiction I have for watching NZ Jetboats  on Youtube!

 

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1 hour ago, Sail4beer said:

I believe it was a tounge in cheek reference to an ancient technology.

Just that when you wanted reverse or to slow down the idea of forcing a reverse of thrust was similar in the most basic concept possible

 

And don’t get me started about the addiction I have for watching NZ Jetboats  on Youtube!

 

Sure looks like that boat flexes a lot, needs more backstay.

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On 11/25/2018 at 2:06 PM, Priscilla said:

Laurie Davidson Pelorus.

Even the maestro can have a off day.

69E422A3-4A31-444E-9BCB-80EBE84D7673.jpeg.96b60d1b1197dca5a598b86d9c659d6d.jpeg

Actually her hull lines are pretty. Shame about the accommodation. 

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2 hours ago, Sail4beer said:

I believe it was a tounge in cheek reference to an ancient technology.

Just that when you wanted reverse or to slow down the idea of forcing a reverse of thrust was similar in the most basic concept possible

 

And don’t get me started about the addiction I have for watching NZ Jetboats  on Youtube!

 

;) Love the supercharging whine at the start of that run but it’s mainly deep, flat & fast - very fast!

Def’ search for skinny water or gorge vids, those boys know how to helm a boat! Gets you into some lovely bits too.

 

 

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13 minutes ago, HFC Hunter said:

;) Love the supercharging whine at the start of that run but it’s mainly deep, flat & fast - very fast!

Def’ search for skinny water or gorge vids, those boys know how to helm a boat! Gets you into some lovely bits too.

 

 

Holy crap. How thick are the bottoms on those boats?

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Ususally just sheet alu. The boats are pretty light. Edit: 5mm, with maybe 10mm at your delta.

The classics are 12mm ply with a glass skin, sometimes Kevlar.

The clever ones also bond a skin of hdpe on for extra slidiness when it gets toooo thin.

Edited by HFC Hunter
Forgot numbers

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37 minutes ago, HFC Hunter said:

Ususally just sheet alu. The boats are pretty light. Edit: 5mm, with maybe 10mm at your delta.

The classics are 12mm ply with a glass skin, sometimes Kevlar.

The clever ones also bond a skin of hdpe on for extra slidiness when it gets toooo thin.

I noticed he went some places where the water was much thinner than the rocks.

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2 hours ago, Ishmael said:

Sure looks like that boat flexes a lot, needs more backstay.

I think the apparent flex is an artifact of the way the camera scans. 

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1 minute ago, Ishmael said:

I noticed he went some places where the water was much thinner than the rocks.

As long as you can keep water coming into the intake and some forward momentum you can get away with a lot. Otherwise you step out and push.

The competition jetsprints are mad: dragster acceleration, with high G turns, in a foot of water. The naviguesser works rather hard!

 

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2 hours ago, HFC Hunter said:

The clever ones also bond a skin of hdpe on

How?

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1 hour ago, SloopJonB said:

How?

Skip to 3.15, but worth checking out the full clip.

 

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3 hours ago, SemiSalt said:

I think the apparent flex is an artifact of the way the camera scans. 

Well, that spoils all the fun.

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13 hours ago, Zonker said:

The crossover point is generally accepted to be around 25-30 knots. Above that range, the extra drag of a rotating prop shaft, P bracket, and rudder starts to really count. Below that range, the smaller impeller of a waterjet (and internal duct resistance) makes waterjets less efficient.

I also like surface drives for very fast boats. Really good efficiencies because you're turning a big prop and rudder chores are handled by swinging drive left/right (though some have external rudders). Not so good in reverse or at slow speeds where the waterjets still work OK

arneson-drive-on-manta.jpg

Thanks for the qualified answer!

I've always wondered whether one could strap a jetski waterjet unit to a small diesel (redneck engineering style!) to power a relatively small boat suitable for fishing bass, retrieving lobster pots and also for the odd time you need to support a scuba diver. I imagine a relatively slender say 25ft plywood boat - just wide enough to retrieve pots safely - obviously a big plus of the waterjet is that there is no propeller in the water, doing 12 knots would be good enough. From what you say the boat would be too slwo for fuel economy to be on the list of advantages!

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7 hours ago, Priscilla said:

Pretty is as pretty does. That boat was designed for a family to economically enjoy safe sailing holidays in Kiwi conditions: sun, wind, rain or storms. LD nailed it.

There’s a complement in NZ to yachts like her: “She’d be a good Sounds boat.” (“Pelorus” being one of our marine Sounds.) Starts to epitomise the beauty in workboats éthique, but in family sailing.

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17 hours ago, Sailbydate said:
On 11/24/2018 at 5:06 PM, Priscilla said:

Laurie Davidson Pelorus.

Even the maestro can have a off day.

69E422A3-4A31-444E-9BCB-80EBE84D7673.jpeg.96b60d1b1197dca5a598b86d9c659d6d.jpeg

Actually her hull lines are pretty. Shame about the accommodation. 

I'd hit that in a second. Then again, I did own and enjoy a WWP19 for a few years so maybe I'm an outlier.

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I don't mind that Pelorus. It ain't pretty but in that JEEP sense of utilitarian styling it works for my eye. I would make the cove stripe more delicate and stop it shorter of the ends. I'd give the boat a tapered bootstripe, not too wide. I'd paint the deck a very light grey/green or grey/blue or maybe a very light tan. I'd use dark curtains in those big windows. The more I look at it the more I like it. That's a sweet hull. Laurie has a very good eye. Not sure how much he got involved with decks and cabin trunks. Some thing tells me not much.

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I finally had time to peruse this thread and it's a great illustration of form vs function, with the above a great example.  And then there are the minor changes as Bob just mentioned that make a big difference in the row away factor.  I still enjoy going fast but now appreciate doing it in comfort.  At the end of the day if it doesn't sail well nothing else matters - like the hot woman with zero personality.

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Crikey, the more I look at the Pelorus I find myself self disagreeing with myself.

It was in reality a forerunner to this hottie with an abundance of personality.

Forgive me Laurie for I have sinned.

49E2F54C-5527-4B32-B2D7-1DF0ACBDFC9F.thumb.jpeg.d680d29ea8e935cd3999b0bf6d6354bb.jpeg

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On 12/8/2018 at 2:35 AM, Panoramix said:

've always wondered whether one could strap a jetski waterjet unit to a small diesel (redneck engineering style!) to power a relatively small boat suitable for fishing bass, retrieving lobster pots and also for the odd time you need to support a scuba diver. I imagine a relatively slender say 25ft plywood boat - just wide enough to retrieve pots safely - obviously a big plus of the waterjet is that there is no propeller in the water, doing 12 knots would be good enough. From what you say the boat would be too slwo for fuel economy to be on the list of advantages!

The short answer is "not a great idea"

Long answer - prop efficiency is proportional to diameter. Big slow turning props (think freighters) are very efficient at providing thrust. Small props have to turn faster to generate thrust but run into tip speed issues (> speed of sound for air propellers; back cavitation for water propellers). So a jetski moving very fast uses a tiny impeller so tip speed doesn't get too high, but that same prop is pretty poor at pushing any significant hull. 

Jet outboards (either factory conversions or aftermarket) are a thing. But while good for really shallow water, they are only about 2/3 as efficient as a good outboard prop. I'd consider those before a jet ski conversion because the impeller diameter will be a lot bigger than a jetski - and lots less engineering to make it work. It's just an outboard with a modified lower section. 

Waterjet bottom shapes have to be carefully designed to avoid getting air bubbles into the jet intake.

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And I like the Pelorus. It looks.... "pesky". I think it would look a bit better if the larger windows were about ~80% of the size they are right now. 

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Pesky is a perfect description of its look.

Maybe the bottom of the windows should run along a line extended from the coamings?

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I totally agree Zonks. Bring the bottom of the big window up until it's in line with the lower edge of the forward ports. Drop the top edge 2". Presto change-o!

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4 hours ago, Zonker said:

The short answer is "not a great idea"

Long answer - prop efficiency is proportional to diameter. Big slow turning props (think freighters) are very efficient at providing thrust. Small props have to turn faster to generate thrust but run into tip speed issues (> speed of sound for air propellers; back cavitation for water propellers). So a jetski moving very fast uses a tiny impeller so tip speed doesn't get too high, but that same prop is pretty poor at pushing any significant hull. 

Jet outboards (either factory conversions or aftermarket) are a thing. But while good for really shallow water, they are only about 2/3 as efficient as a good outboard prop. I'd consider those before a jet ski conversion because the impeller diameter will be a lot bigger than a jetski - and lots less engineering to make it work. It's just an outboard with a modified lower section. 

Waterjet bottom shapes have to be carefully designed to avoid getting air bubbles into the jet intake.

The fishing boats on Pt Stephens bay (~70nm N of Sydney) still hand broadcast their nets from rowing boats (local design, about 16'). Several of them have been converted to engine, using old jetskis - everything but the seats - mounted amidships.  Presumably more concerned with maneuverability in the shallows and not fouling their nets than speed. Seem to work OK.

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12 hours ago, Zonker said:

The short answer is "not a great idea"

Long answer - prop efficiency is proportional to diameter. Big slow turning props (think freighters) are very efficient at providing thrust. Small props have to turn faster to generate thrust but run into tip speed issues (> speed of sound for air propellers; back cavitation for water propellers). So a jetski moving very fast uses a tiny impeller so tip speed doesn't get too high, but that same prop is pretty poor at pushing any significant hull. 

Jet outboards (either factory conversions or aftermarket) are a thing. But while good for really shallow water, they are only about 2/3 as efficient as a good outboard prop. I'd consider those before a jet ski conversion because the impeller diameter will be a lot bigger than a jetski - and lots less engineering to make it work. It's just an outboard with a modified lower section. 

Waterjet bottom shapes have to be carefully designed to avoid getting air bubbles into the jet intake.

Thanks, I kind of feared that would be the answer as intutively the big prop makes sense!

7 hours ago, Fleetwood said:

The fishing boats on Pt Stephens bay (~70nm N of Sydney) still hand broadcast their nets from rowing boats (local design, about 16'). Several of them have been converted to engine, using old jetskis - everything but the seats - mounted amidships.  Presumably more concerned with maneuverability in the shallows and not fouling their nets than speed. Seem to work OK.

Interesting, have you got photos?

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On 12/8/2018 at 9:18 PM, Priscilla said:

Bob, she has been listed on SA classified for quite a few moons.

If I was not in the land of the long white cloud I would add her to my fleet in a heartbeat.

6F9E5781-65AB-42C1-B19B-82C1F1CF0AB8.thumb.jpeg.ad323bad4668f9fbe7a021819dce3316.jpeg

 

https://sailinganarchy.com/classifieds/show-ad/?id=3827

 

Built in 1984, those Kiwi designers rock!

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8 hours ago, Fleetwood said:

The fishing boats on Pt Stephens bay (~70nm N of Sydney) still hand broadcast their nets from rowing boats (local design, about 16'). Several of them have been converted to engine, using old jetskis - everything but the seats - mounted amidships.  Presumably more concerned with maneuverability in the shallows and not fouling their nets than speed. Seem to work OK.

Which one is it?

DA1BAAFB-7393-468A-B2AC-B4B0504E02EB.jpeg

C5FFBFEC-262D-4DF3-A012-B3AC52F8CE4C.jpeg

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17 hours ago, Zonker said:

Hey Fleetwood - any idea what sort of speed they get from these jetski engines?

Never seen them in action; I'm usually still tucked up in bed when they are out!   I'd be surprised if they go much faster than hull speed, they're workboats after all and the bay is only a couple of miles wide by maybe 5 miles deep.

I'll get some pics next time I'm there.

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"An innovative use of integrated sponsons and aggressive spray knockers result in an efficient design in both calm and rougher conditions."

What's a 'spray knocker' , Bob?

 

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Tricky;

Most powerboats have some detail at the chine to deflect the spray. It can be a small, molded in, horizontal "flat' running the length of the chine, this is common.. Or it ca be an added  strip shaped with a flat bottom just big enough to deflect the spray. When I tank tested the model for my 42' powerboat I tried it with and without the spray knocker. I was interested in finding out exactly how far forward I needed to run the spray knocker. With no spray knock the boat was impossible. Water just peeled up the topsides and looked like it would shower into the cockpit area.

On my  Lobster boat inspired powerboat the spray knocker is clearly visible forward.

44191559542_88cb0edf04_k.jpgLobby by robert perry, on Flickr

On this shot of the 42' double ended powerboat plug you can clearly see the built in flat at the chine to act as a spray knocker.

46222231752_f709394293_k.jpg42mold2 by robert perry, on Flickr

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Smart rail makes them as aftermarket products to attach near the chine. 

https://www.integritymarinecorp.com/smart-rails

I’m getting them for a 20’ Shamrock restoration/diesel conversion. I think they’ll work just fine. It’s almost as wet a ride now as a Chris Craft dory.

85154612-1495-4F13-A669-31A51749EDE1.jpeg

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