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      A Few Simple Rules   05/22/2017

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    • B.J. Porter

      Moderation Team Change   06/16/2017

      After fifteen years of volunteer moderation at SA, I will no longer be part of the moderation team. The decision to step aside is mine, and has been some time in the works but we did not wish to announce it in advance for a number of reasons. It's been fun, but I need my time back for other purposes now. The Underdawg admin account will not be monitored until further notice, as I will be relinquishing control of it along with my administrative privileges. Zapata will continue on as a moderator, and any concerns or issues can be directed to that account or to the Editor until further notice. Anyone interested in helping moderate the forums should reach out to Scot by sending a PM to the Editor account. Please note that I am not leaving the community, I am merely stepping aside from Admin responsibilities and privileges on the site.

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Trickypig

Coefficient of drag in water.

46 posts in this topic

More and more keelboat designs are becoming planing machines. Flying assymetric spinnakers is becoming faster on many boats even in light to moderate breezes than symmetrical spinnakers.

 

I'm not a boat designer's bootlace.. but now that our boats are planing more often than not, we seem to have too much drag in the forms of propulsion we fit to our keelboats and the same goes for multihulls.

 

We have low displacement, seamless sails, narrow foils, very little rocker in the hulls, etc etc, but we still have to drag around some ironmongery under the hull that at speeds of over 10 knots must have huge amounts of drag.

 

Is anyone developing an affordable `no drag' drive system that'll satisfy Cat 1 and not cost the equivalent of a sail wardrobe?

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More and more keelboat designs are becoming planing machines. Flying assymetric spinnakers is becoming faster on many boats even in light to moderate breezes than symmetrical spinnakers.

 

I'm not a boat designer's bootlace.. but now that our boats are planing more often than not, we seem to have too much drag in the forms of propulsion we fit to our keelboats and the same goes for multihulls.

 

We have low displacement, seamless sails, narrow foils, very little rocker in the hulls, etc etc, but we still have to drag around some ironmongery under the hull that at speeds of over 10 knots must have huge amounts of drag.

 

Is anyone developing an affordable `no drag' drive system that'll satisfy Cat 1 and not cost the equivalent of a sail wardrobe?

 

Magnetohydrodynamics is the answer.

 

http://en.wikipedia....rodynamic_drive

 

Not sure about cost and would probably drain your 24V house battery pretty quick though...sorry!

 

Try this for starters!

 

http://forums.sailin...opic=71324&st=0

 

DW

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Living doll a farr 55 has a retractable prop shaft to reduce drag and is cat 1 compliant.

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Living doll a farr 55 has a retractable prop shaft to reduce drag and is cat 1 compliant.

 

Who built it?

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There's this, from here: http://www.boatspeed...ages/shop.shtml

 

http://www.boatspeed.com.au/pages/movie.shtml

 

(Not sure about the door)

 

Then there's this, from here: http://www.amartech.nl/products/retractable-propulsion

 

post-419-036251900 1333026185_thumb.jpg

 

 

And this

 

al%20fresco.jpg

 

 

has this:

 

al%20fresco%20prop.jpg

 

 

 

The R/P Cone of Silence had a ducted impeller system, IIRC.

 

OTOH, I have seen a proposal by a builder of VO70s that they get rid of the retracting prop, as it causes too much hassle, expense and complication for limited gain. ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

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Living doll a farr 55 has a retractable prop shaft to reduce drag and is cat 1 compliant.

 

Who built it?

Was it Cooksons

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al%20fresco%20prop.jpg

 

 

Not talking about practical issues, as I'm unqualified to judge, but just as a personal take: That's sweet.

 

 

ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

 

 

And that's food for thought.

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The Rambler 90 (ex Alfa) has a tunnel drive, it also is the ballast pump for the tanks. Front and back doors open=propulsion, front door open back door closed=water into tanks, front door closed back door open=water out.

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Living doll a farr 55 has a retractable prop shaft to reduce drag and is cat 1 compliant.

 

Who built it?

Was it Cooksons

 

Yeah Living Doll, Farr 55, built at Cooksons 2008.

 

Clicky

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The R/P Cone of Silence had a ducted impeller system, IIRC.

 

OTOH, I have seen a proposal by a builder of VO70s that they get rid of the retracting prop, as it causes too much hassle, expense and complication for limited gain. ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

 

I'm thinking that as boats get larger and more powerful the contributing drag of what amounts to the same size saildrive becomes proportionally less. Although I would have thought the 60s (both multi and mono) would benefit. It may be a rule thing.

 

However as the boats get smaller so the relative drag becomes greater. Having sailed on the Cone I've no doubt that its downwind speed was because of the `doors'. It satified the offshore safety requirements but no drag. The Tow Truck boys were always `towing' their saildrive with them in their Mumm30.

 

In the end it comes down to cost and I still have no handle on that..

post-14496-019688500 1333060824_thumb.jpg

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Retractable also allows nice big efficient 3 bladed prop so faster motoring or a smaller lighter motor

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Not sure about the boatspeed version - too many moving parts. Agree the red one looks sweet. Just up and down and big fixed 3 blade prop. Extra drag from the bottom flat panel would not be a big issue. Preferable to a door not opening/ closing properly. Potential for a lot of sideways force on the door hinges too. Can imagine that alignment could be a problem over time. Can't always be dead stopped when you open the door.

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The R/P Cone of Silence had a ducted impeller system, IIRC.

 

OTOH, I have seen a proposal by a builder of VO70s that they get rid of the retracting prop, as it causes too much hassle, expense and complication for limited gain. ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

 

I'm thinking that as boats get larger and more powerful the contributing drag of what amounts to the same size saildrive becomes proportionally less. Although I would have thought the 60s (both multi and mono) would benefit. It may be a rule thing.

 

However as the boats get smaller so the relative drag becomes greater. Having sailed on the Cone I've no doubt that its downwind speed was because of the `doors'. It satified the offshore safety requirements but no drag. The Tow Truck boys were always `towing' their saildrive with them in their Mumm30.

 

In the end it comes down to cost and I still have no handle on that..

 

Fuck I hated that system.

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The R/P Cone of Silence had a ducted impeller system, IIRC.

 

OTOH, I have seen a proposal by a builder of VO70s that they get rid of the retracting prop, as it causes too much hassle, expense and complication for limited gain. ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

 

I'm thinking that as boats get larger and more powerful the contributing drag of what amounts to the same size saildrive becomes proportionally less. Although I would have thought the 60s (both multi and mono) would benefit. It may be a rule thing.

 

However as the boats get smaller so the relative drag becomes greater. Having sailed on the Cone I've no doubt that its downwind speed was because of the `doors'. It satified the offshore safety requirements but no drag. The Tow Truck boys were always `towing' their saildrive with them in their Mumm30.

 

In the end it comes down to cost and I still have no handle on that..

 

Fuck I hated that system.

 

You never really did fit below to well Big Unit. Funny how you got the job of closing it.

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Not sure about the boatspeed version - too many moving parts. Agree the red one looks sweet. Just up and down and big fixed 3 blade prop. Extra drag from the bottom flat panel would not be a big issue. Preferable to a door not opening/ closing properly. Potential for a lot of sideways force on the door hinges too. Can imagine that alignment could be a problem over time. Can't always be dead stopped when you open the door.

 

I like that one two.. I can imagine there's a strut in a guide down near the propeller end. If you have a uni joint on the coupling there wouldn't be alignment issues with the shaft. I think the plate would pull back into its seat too.

 

Pity someone can't go into production of them as a drop in unit.. just add piece of hull cut out.

 

I wonder what the potential market would be? I know everyone is gonna say "not big". 200/year 500/year?

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The R/P Cone of Silence had a ducted impeller system, IIRC.

 

OTOH, I have seen a proposal by a builder of VO70s that they get rid of the retracting prop, as it causes too much hassle, expense and complication for limited gain. ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

 

I'm thinking that as boats get larger and more powerful the contributing drag of what amounts to the same size saildrive becomes proportionally less. Although I would have thought the 60s (both multi and mono) would benefit. It may be a rule thing.

 

However as the boats get smaller so the relative drag becomes greater. Having sailed on the Cone I've no doubt that its downwind speed was because of the `doors'. It satified the offshore safety requirements but no drag. The Tow Truck boys were always `towing' their saildrive with them in their Mumm30.

 

In the end it comes down to cost and I still have no handle on that..

 

Fuck I hated that system.

I never saw Cone do more than about three knots in flat water with that system (when it worked that is) so question whether it complied with special regs? Or was it just that you guys enjoyed leasurely sails to and from races when everyone else was back at the bar?

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The R/P Cone of Silence had a ducted impeller system, IIRC.

 

OTOH, I have seen a proposal by a builder of VO70s that they get rid of the retracting prop, as it causes too much hassle, expense and complication for limited gain. ORMA 60 tris and IMOCA Open 60s are happy with saildrives.

 

I'm thinking that as boats get larger and more powerful the contributing drag of what amounts to the same size saildrive becomes proportionally less. Although I would have thought the 60s (both multi and mono) would benefit. It may be a rule thing.

 

However as the boats get smaller so the relative drag becomes greater. Having sailed on the Cone I've no doubt that its downwind speed was because of the `doors'. It satified the offshore safety requirements but no drag. The Tow Truck boys were always `towing' their saildrive with them in their Mumm30.

 

In the end it comes down to cost and I still have no handle on that..

 

Fuck I hated that system.

I never saw Cone do more than about three knots in flat water with that system (when it worked that is) so question whether it complied with special regs? Or was it just that you guys enjoyed leasurely sails to and from races when everyone else was back at the bar?

 

 

Trust me - it complied. We were subject to "random" safety checks for every friggin east coast race we ever did.

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All the V70s have retractable shaft/prop assemblies with universal couplings and thrust bearings. Pretty simple mechanisms and operation.

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All the V70s have retractable shaft/prop assemblies with universal couplings and thrust bearings. Pretty simple mechanisms and operation.

 

 

this is a practical solution for a larger boats/budgets but for smaller boats weight and cost will bite you in the arse, that thing in the cone was pretty crude and user unfreindly

but it weighed probably 20kg as opposed to say 170kg for a deisel and retractable shaft skeg gismo, maybe with some development and refinement it could come good.

perhaps a jet ski pump with the outboard gearbox and powerhead driving it. a power head that is fuel injected would also be a good thing.

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How is this for an adaptation - jetski in a flat bottomed tinnie - Kiwi ingenuity or madness

post-720-049660500 1333094179_thumb.jpg

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How is this for an adaptation - jetski in a flat bottomed tinnie - Kiwi ingenuity or madness

 

 

looks like the whole familys out for a ride bro.

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The good news is they inevitably thrust up right in the middle of where the nav seat wants to go. makes for a tidy spot to place your beer while " checking the charts"

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How is this for an adaptation - jetski in a flat bottomed tinnie - Kiwi ingenuity or madness

 

Wouldn't it have been more practical to extract the drive unit from the jetski chassis?

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The new Columbia 32 Carbon has a retractable 3 blade unit:

 

post-442-024383100 1333126142_thumb.jpg post-442-074963700 1333126189_thumb.jpg

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Execution looks sweet, prop doesn't. I think someone said it was from a torquedo.

 

 

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Execution looks sweet, prop doesn't. I think someone said it was from a torquedo.

Ha, ha.

Prop looks sweet, execution doesn't- imho.

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The new Columbia 32 Carbon has a retractable 3 blade unit:

 

post-442-024383100 1333126142_thumb.jpg post-442-074963700 1333126189_thumb.jpg

 

Farr 11s up here has one of those. Uses a folding two-blade prop instead of a fixed blade prop. Don't know the thinking behind it but the boat has all the fancy hardware (canting, twin boards etc.) so it would be interesting to see the justification.

 

HW

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The new Columbia 32 Carbon has a retractable 3 blade unit:

 

post-442-024383100 1333126142_thumb.jpg post-442-074963700 1333126189_thumb.jpg

 

Farr 11s up here has one of those. Uses a folding two-blade prop instead of a fixed blade prop. Don't know the thinking behind it but the boat has all the fancy hardware (canting, twin boards etc.) so it would be interesting to see the justification.

 

HW

 

I searched everywhere for info on this boat's drive system and found nothing except Bob Perry reviewing the design in its early conception as having a lifting 15hp outboard. It would seem there would be no need to have a folding prop on a retractable system and it may actually be less efficient.

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The new Columbia 32 Carbon has a retractable 3 blade unit:

 

post-442-024383100 1333126142_thumb.jpg post-442-074963700 1333126189_thumb.jpg

 

Farr 11s up here has one of those. Uses a folding two-blade prop instead of a fixed blade prop. Don't know the thinking behind it but the boat has all the fancy hardware (canting, twin boards etc.) so it would be interesting to see the justification.

 

HW

 

I searched everywhere for info on this boat's drive system and found nothing except Bob Perry reviewing the design in its early conception as having a lifting 15hp outboard. It would seem there would be no need to have a folding prop on a retractable system and it may actually be less efficient.

 

i think the folding prop cavity caries less water when stowed,

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I'd like designer's comments on this. Put one of these in the bulb of a keel, run the shaft out the back, and add one of these at the trailing end of the keel. Batteries and controller stay in the hull.

 

Specs on the moter http://www.proteanelectric.com/?page_id=158&post=229

 

well I considered that solution on a boat a few years back - ideal place for the prop, almost... Big drawback is if you are aground and want to back off!

technically its easy by various methods, another drawback is that you've lost some keel density, so for the same righting moment you've got to have a bigger bulb volume, and again thats not what you want for performance.

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The new Columbia 32 Carbon has a retractable 3 blade unit:

 

post-442-024383100 1333126142_thumb.jpg post-442-074963700 1333126189_thumb.jpg

 

Farr 11s up here has one of those. Uses a folding two-blade prop instead of a fixed blade prop. Don't know the thinking behind it but the boat has all the fancy hardware (canting, twin boards etc.) so it would be interesting to see the justification.

 

HW

 

I searched everywhere for info on this boat's drive system and found nothing except Bob Perry reviewing the design in its early conception as having a lifting 15hp outboard. It would seem there would be no need to have a folding prop on a retractable system and it may actually be less efficient.

 

i think the folding prop cavity caries less water when stowed,

 

I would have thought the simpler, lighter and even alloy fixed prop would require a cavity that carried only, say, half a gallon more so it would pretty much a wash when we are talking about weight low in the boat. Also, I don't understand how you fold the prop reliably before pulling it up. Does it mean the old speed her up and `kill the donk' method of folding the prop?

 

Has anyone seen this boat up close, out of the water?

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I think the idea is that the gaps between the hull and retracted cover that are created longitudinally don't add a lot of drag, but the gaps athwartship do (relatively speaking).

 

I think you could use a fixed two blade in this app, as long as you kept the blades vertical when retracting. You would probably get a little more thrust out of a fixed over a folder, and it should be slightly lighter too.

 

I always wondered if you were going to do this like the Columbia did, why you wouldn't fit a Kort Nozzle around the prop to make it more efficient? I mean other than having to make a custom prop and building and attaching the nozzle.

 

 

 

BTW, I started a post about this a while back, with a decent photo of one of the VO70's with one.

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I think the idea is that the gaps between the hull and retracted cover that are created longitudinally don't add a lot of drag, but the gaps athwartship do (relatively speaking).

 

I think you could use a fixed two blade in this app, as long as you kept the blades vertical when retracting. You would probably get a little more thrust out of a fixed over a folder, and it should be slightly lighter too.

 

I always wondered if you were going to do this like the Columbia did, why you wouldn't fit a Kort Nozzle around the prop to make it more efficient? I mean other than having to make a custom prop and building and attaching the nozzle.

 

 

 

BTW, I started a post about this a while back, with a decent photo of one of the VO70's with one.

 

Thanks for the link to the thread.

 

Every system shown seems to be a semi custom fabrication. I'm wondering if the market is at a point were someone could specialize in this one system and get the costs down. Since a saildrive is a fairly complex piece of engineering, so a retractable system if built in numbers may be also possible and attractive to owners of faster boats.

 

You'd have to make a study of the advantages of a Kort nozzle. It appears their advantage is in low speed high thrust applications. The only drawback of the retractable yacht shaft is that the Kort nozzle will be at a downwards angle in this configuration.

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I think the idea is that the gaps between the hull and retracted cover that are created longitudinally don't add a lot of drag, but the gaps athwartship do (relatively speaking).

 

I think you could use a fixed two blade in this app, as long as you kept the blades vertical when retracting. You would probably get a little more thrust out of a fixed over a folder, and it should be slightly lighter too.

 

I always wondered if you were going to do this like the Columbia did, why you wouldn't fit a Kort Nozzle around the prop to make it more efficient? I mean other than having to make a custom prop and building and attaching the nozzle.

 

 

 

BTW, I started a post about this a while back, with a decent photo of one of the VO70's with one.

 

Thanks for the link to the thread.

 

Every system shown seems to be a semi custom fabrication. I'm wondering if the market is at a point were someone could specialize in this one system and get the costs down. Since a saildrive is a fairly complex piece of engineering, so a retractable system if built in numbers may be also possible and attractive to owners of faster boats.

 

You'd have to make a study of the advantages of a Kort nozzle. It appears their advantage is in low speed high thrust applications. The only drawback of the retractable yacht shaft is that the Kort nozzle will be at a downwards angle in this configuration.

 

i was looking at a bmw k100 motorcycle engine, thought it could make a good start low c of g, water cooled, fuel injected, lightweight, swinging shaftdrive,cheap,but no reverse.

,,

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I think the idea is that the gaps between the hull and retracted cover that are created longitudinally don't add a lot of drag, but the gaps athwartship do (relatively speaking).

 

I think you could use a fixed two blade in this app, as long as you kept the blades vertical when retracting. You would probably get a little more thrust out of a fixed over a folder, and it should be slightly lighter too.

 

I always wondered if you were going to do this like the Columbia did, why you wouldn't fit a Kort Nozzle around the prop to make it more efficient? I mean other than having to make a custom prop and building and attaching the nozzle.

 

 

 

BTW, I started a post about this a while back, with a decent photo of one of the VO70's with one.

 

Thanks for the link to the thread.

 

Every system shown seems to be a semi custom fabrication. I'm wondering if the market is at a point were someone could specialize in this one system and get the costs down. Since a saildrive is a fairly complex piece of engineering, so a retractable system if built in numbers may be also possible and attractive to owners of faster boats.

 

You'd have to make a study of the advantages of a Kort nozzle. It appears their advantage is in low speed high thrust applications. The only drawback of the retractable yacht shaft is that the Kort nozzle will be at a downwards angle in this configuration.

 

i was looking at a bmw k100 motorcycle engine, thought it could make a good start low c of g, water cooled, fuel injected, lightweight, swinging shaftdrive,cheap,but no reverse.

,,

 

Remember the last time BMW put an engine in a sailboat! They should stick to sweet inline sixes.

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If i were to choose an auto engine to marinize, It would be the smallest Mazda rotary 4 cyl they made... those engines ran smooth & quiet.

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If i were to choose an auto engine to marinize, It would be the smallest Mazda rotary 4 cyl they made... those engines ran smooth & quiet.

 

Apart from the fact that they only made a hand full of 4 rotor engines for their Lemans cars... They need a massive amount of cooling, oil and water, they burn oil (by design) and use large amounts of petrol, and then they are not very light either

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If i were to choose an auto engine to marinize, It would be the smallest Mazda rotary 4 cyl they made... those engines ran smooth & quiet.

 

Apart from the fact that they only made a hand full of 4 rotor engines for their Lemans cars... They need a massive amount of cooling, oil and water, they burn oil (by design) and use large amounts of petrol, and then they are not very light either

 

Nor very durable. Apex seals failing were a common problem. Lack of NVH was probably the wankel engines most redeeming quality.

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If i were to choose an auto engine to marinize, It would be the smallest Mazda rotary 4 cyl they made... those engines ran smooth & quiet.

 

Apart from the fact that they only made a hand full of 4 rotor engines for their Lemans cars... They need a massive amount of cooling, oil and water, they burn oil (by design) and use large amounts of petrol, and then they are not very light either

 

Nor very durable. Apex seals failing were a common problem. Lack of NVH was probably the wankel engines most redeeming quality.

 

 

if i were to try and put a prototype together for say 28 to 38ft race yacht what would you all like to see, a swing shaft type or some kind of jet drive fashioned around a jetski waterblaster or something else?

 

what kind of power head petrol(much lighter & cheaper) or deisel, what horsepower?what would be a resonable price to pay for a low drag bolt in settup?

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if i were to try and put a prototype together for say 28 to 38ft race yacht what would you all like to see, a swing shaft type or some kind of jet drive fashioned around a jetski waterblaster or something else?

 

what kind of power head petrol(much lighter & cheaper) or deisel, what horsepower?what would be a resonable price to pay for a low drag bolt in settup?

 

The Cone has a Tohatsu 2 stroke outboard(w/o leg) on a custom built jet drive setup. It's very light. All the jetski motors appear to have a lot more horsepower (30hp to 250hp) and I imagine would be heavier.

 

I've looked around the www and I'm yet to find a schematic of a jetski motor with a weight stated for it as a component including ducts.

 

The doors/tunnel would need to be a good design. Cone had so little hull rocker or boat under the water that we just carried that little amount of water over each door with us. The design was to have had bags we inflated to expel water when the doors were shut, but they didn't work. The doors perhaps could have been thicker foamed blocks and cavities made to accommodate them.

 

Another design idea would be to have the aft(jet)door as a hinged component made up of some hull and transom (as one right angled piece) hinged at the top. That way the jet can be angled correctly and a jetski motor could be fitted with only one door forward for the intake needed.

 

Having suggested all of that, I think the retractable prop and shaft is the better solution as it is already designed, but needs to simply be finessed for small scale production line manufacture. The biggest advancement will be how to make it available to the boatbuilder as a `drop in package'.

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I'd like designer's comments on this. Put one of these in the bulb of a keel, run the shaft out the back, and add one of these at the trailing end of the keel. Batteries and controller stay in the hull.

 

Specs on the moter http://www.proteanel...id=158&post=229

 

Didn't "Big fun" or one of those big IOR boats around here in the 80's have vertical drive shaft through the keel. right angle drive out to a prop (2 blade feather? folder?) out the trailing edge of the keel? engine in cabin- another 90 degree? or vertical crankshaft.

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I think the idea is that the gaps between the hull and retracted cover that are created longitudinally don't add a lot of drag, but the gaps athwartship do (relatively speaking).

I think you could use a fixed two blade in this app, as long as you kept the blades vertical when retracting. You would probably get a little more thrust out of a fixed over a folder, and it should be slightly lighter too.

I always wondered if you were going to do this like the Columbia did, why you wouldn't fit a Kort Nozzle around the prop to make it more efficient? I mean other than having to make a custom prop and building and attaching the nozzle.

BTW, I started a post about this a while back, with a decent photo of one of the VO70's with one.

 

Theres a multihull with that 2 blade approach- might even have been feathering

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I'd like designer's comments on this. Put one of these in the bulb of a keel, run the shaft out the back, and add one of these at the trailing end of the keel. Batteries and controller stay in the hull.

 

Specs on the moter http://www.proteanel...id=158&post=229

 

Didn't "Big fun" or one of those big IOR boats around here in the 80's have vertical drive shaft through the keel. right angle drive out to a prop (2 blade feather? folder?) out the trailing edge of the keel? engine in cabin- another 90 degree? or vertical crankshaft.

 

Putting the prop immediately behind a foil used to work until the foils got so high aspect and thin. The prop can't effectively hide from the now cleaner and faster water passing by the foils.

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if i were to try and put a prototype together for say 28 to 38ft race yacht what would you all like to see, a swing shaft type or some kind of jet drive fashioned around a jetski waterblaster or something else?

 

what kind of power head petrol(much lighter & cheaper) or deisel, what horsepower?what would be a resonable price to pay for a low drag bolt in settup?

 

The Cone has a Tohatsu 2 stroke outboard(w/o leg) on a custom built jet drive setup. It's very light. All the jetski motors appear to have a lot more horsepower (30hp to 250hp) and I imagine would be heavier.

 

I've looked around the www and I'm yet to find a schematic of a jetski motor with a weight stated for it as a component including ducts.

 

The doors/tunnel would need to be a good design. Cone had so little hull rocker or boat under the water that we just carried that little amount of water over each door with us. The design was to have had bags we inflated to expel water when the doors were shut, but they didn't work. The doors perhaps could have been thicker foamed blocks and cavities made to accommodate them.

 

Another design idea would be to have the aft(jet)door as a hinged component made up of some hull and transom (as one right angled piece) hinged at the top. That way the jet can be angled correctly and a jetski motor could be fitted with only one door forward for the intake needed.

 

Having suggested all of that, I think the retractable prop and shaft is the better solution as it is already designed, but needs to simply be finessed for small scale production line manufacture. The biggest advancement will be how to make it available to the boatbuilder as a `drop in package'.

 

250 hp would make for some quick deliveries,i had a brief look at a jet ski pump seems to run a small impeller at the front into an expansion chamber where the water spun against pitched blades fixed to the output housing.i think the pressure in the pump

is created by centrafuge rather than thrust.

i think the pump concept could hold a more compact bolt in solution, more practicle for retro fit and has more possibility for a turn the key and go outcome. it could also be fitted to any kind of boat.

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