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Repowering (?) how much HP?


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#1 Anomaly2

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Posted 14 November 2012 - 12:14 AM

So, I'm thinking about re-powering. Wait, make that I'm "considering" re-powering. Moe knows what I'm thinking about. Anyway, where was I? Oh yeah, so my old Perkins 4108 is showing some signs of wear and while I"m in no way committed to re-powering, I'm at least considering it in the sense of weighing up the tradeoffs between putting money into the Perkins vs taking the plunge. One big question I have is how much HP is enough? The boat is 33' LOA by 9'6" beam and displaces 11,000-12,000 lbs (that's just a WAG, Bob would certainly know better....). The Perkins is 50 HP and arguably way more than this boat needs. It certainly makes the boat squat at anchor more than I would like... I have a tiny prop (10") and no room to increase the diameter. The current gear is a 1:1 velvet drive. Lots of the smaller diesels I have looked at (20-25 HP range--- another WAG on my part) have been coupled to 2:1 and even 3:1 gears. which seems to be something else to factor in. But, I'm out of my knowledge range here. Any advice from this crowd would be appreciated.

#2 Zonker

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Posted 14 November 2012 - 12:24 AM

What type of boat? 12" is a very small prop. Is your prop in an aperture and that's why you can't increase it's diameter?

Normally with a 33' x 12,000 lb boat I'd be looking at say 25-30 HP as well (any of the 3 cylinders by Yanmar, Beta, etc etc) and a 16-17" prop for better efficiency. And with this size engine I'd be looking at a gear ratio > 2 to turn a slower moving big prop.

You're sort of in the same boat (no pun intended) as people with Atomic 4's with 1:1 gear turningl itty bitty props quickly. Good for an outboard, not good for a displacement hull.

What sort of speed do you get with current engine, what RPM and what is engine max RPM?

#3 Thorvald

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Posted 14 November 2012 - 12:28 AM

Wow, is there really no way to increase that prop diameter? You didn't tell us what the prop was. If it's a two blade can you go to a three blade Max prop so you can at least match the pitch to your engine/ prop combo? Your size and displacement is a close match to my yawl and I'm going with 25hp. 20 would have been OK but the 25 was on sale at the boat show. I'm using a 14" two blade Max. I was told that a 16" would have been even better.

#4 Anomaly2

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Posted 14 November 2012 - 01:07 AM

Thor and Zonk, thanks but..... when I say there is no room to increase the diameter... I mean there is no room to increase the diameter. Well OK, maybe there is something to discuss here,,,, here's a photo (apologies for the lighting but it was all I could find on this laptop). Sure, it looks like there is SOME room but I'm already getting some interesting prop wash effects from how close the prop is to the hull. The boat is a Soverel 33-1 (the early cruising version, not the one design Soverel 33 that is far better known). I don't know why Soverel built the shaft log the way the did but its what I have to work with. This photo exaggerates the clearance if you ask me. The prop in the picture is the current 10" 3-blade prop (I don't have the figure for the pitch right in front of me at the moment). I seem to recall on the trip up from Florida, the Perkins could push the boat at 6- 6.5 kts at 2000 rpms (with the 1:1 gear). I agree it would be wonderful to turn a bigger wheel slower....

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#5 Scarecrow

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Posted 14 November 2012 - 01:17 AM

Aim for about 10% of the diameter as your tip clearance. You look to have 20-30%

#6 SemiSalt

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Posted 14 November 2012 - 01:17 AM

The very approximate rule of thumb is 4hp/ton, so a 6 ton boat would call for 24 hp. (The book I got this from uses metric units and I believe it works out to 3.7 hp/ton). Of course, if you waste a lot of power with a small prop at high speed, you might need more.

A clear case of needing professional advice about increasing prop diameter.

#7 Bob Perry

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Posted 14 November 2012 - 01:33 AM

15% tip clearance would be generous.
An honest 18 to 20hp would do it.
We live in a world where everyone wants lots of hp but you sure as hell don't need 35 Perkins horses on that boat.
I had the 4-108 on the 37', 17,000lb. Esprit and it was overkill.

#8 Gatekeeper

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Posted 14 November 2012 - 01:46 AM

An

I am running a much larger prop that is only about an inch away from the skeg...I know I get a slight "thump" at certain rpm, but that's about it.

Consider a larger prop. It'll open up lots more options.

That prop is wayyyyy too small. I'm betting you can swing a 14" in there.

As far as repowering I've always been a BIG Yanmar fan, but BETA is impressing me more and more. They have really nice repowering packages.

#9 Anomaly2

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Posted 14 November 2012 - 02:20 AM

Thanks everyone.It would appear I do have more options than I thought. The tip clearance info is great news. On todays high horsepower world- Bob, I've always thought the 4108 was overkill, but of course I used to drive (slowly) a VW poptop van with the original "upright" VW engine. I think that was all of 48hp. You can imagine my thrill when I upgraded to my current daily driver- a 1985 Toyota Tercel T4WD four wheel drive wagon with a whopping 62 hp.

Does a larger prop mean more drag when sailing?

#10 kent_island_sailor

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Posted 14 November 2012 - 02:48 AM

IIRC, Beta can be had with a reduction ratio close to 1:1 if you ask.

#11 Ishmael

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Posted 14 November 2012 - 03:14 AM

You could also consider a Max Prop, which would give you even more room since the blades sit further back on the shaft.

Posted Image

The end of the shaft is in piece 1...

Which corresponds to where the first set of allan screws sit.

Attached File  prop.jpg   233.52K   2 downloads

The prices for Max Props are nowhere as steep as they used to be; I got mine for $700.

#12 slap

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Posted 14 November 2012 - 03:21 AM

For efficiency, you typically want the biggest possible prop rotating at the lowest possible rpm (without cavitating). I figure when I went from a 1:1 ratio atomic bomb to a diesel with a 2:1 reduction ratio, my propeller efficiency went from around 25% up to 35%, at the same diameter.

#13 Diarmuid

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Posted 14 November 2012 - 03:26 AM

If you wanted to roll the dice, here's a new/old BMW 45bhp marine diesel in your neck of the woods. No tranny, alternator, etc. But only $1500.

#14 Balder

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Posted 14 November 2012 - 04:53 AM

I am a huge fan of the Kiwi feathering prop.

http://www.kiwiprop.us/index.htm
http://www.kiwiprop....stallation.html

End user adjustable pitch. And you can carry extra blades so a diver can change blades - no sending out to the propshop if you wack something. Great if you need to experiment with pitch after the repower. It does look like 14" is the small one.

Prices are similar (I think) to maxprop.

Reverses pitch when you go in reverse, the couple boats I have used with them had zero propwalk

#15 Balder

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Posted 14 November 2012 - 05:29 AM

Also (they claim, I have no first hand experience with this) they require even less than the 10% minimum clearance



[font='Times New Roman', ', Times, serif} ']12: How much tip clearance is required?To obtain clean water away from the water that is dragged forward along with the hull and impacts on propeller performance all propellers require clearance from the tips to the hull. Having low tip clearances can also cause vibration to be transmitted through the hull from the disturbance caused by the propeller blades displacing the incompressible water as they sweep past the hull. Some rules of thumb use 10 % of the diameter - but higher clearances generally lead to higher shaft angles which reduces propeller performance. Due to the low boat speeds typically involved with low powered displacement yachts - clearance is not as critical as on many applications. With thin tips, unlike folding propellers which use the mass in the tips to provide reverse thrust from the centrifugal force generated, our empirical experience is that clearance can be lower with virtually no impact on performance or vibration. We would suggest no less than ½" or 12 mm in a tight situation - obviously more is better.[/font]

#16 Gatekeeper

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Posted 14 November 2012 - 02:43 PM

Prices are similar (I think) to maxprop.


I think you'll find they are half the price of the Max...or less. Great product.

We push GK with a 22hp engine and I have never felt she lacked for power, but she is a bit lighter than than your Soverel, but I would guess much above 26 hp would be a waste.

#17 Bob Perry

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Posted 14 November 2012 - 02:43 PM

I would not go less than 10%. I design for at least 15%. Given the low deadrise shape of Anom's bottom at that point I would go with max clearance if possible.
Obviously he's not going to be changing his shaft angle.

#18 Anomaly2

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Posted 14 November 2012 - 02:54 PM

Hmmm, found another photo--- this time with a tape measure conveniently deployed (though not quite in the right position). Perhaps I was thinking about this even when I didn't know I was.... If you zoom the photo, you can make out that the current prop is stamped "11P x 10" and from the tape measure it would seem that a 14" prop would fit but with less than 10% clearance.

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#19 Gatekeeper

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Posted 14 November 2012 - 03:03 PM

I'm betting the KIWI would work in there...with the blades on them being much further aft you will gain clearance.

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#20 Becalmed

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Posted 14 November 2012 - 03:04 PM

It would be great to know the exact clearance, and inch of diameter is pretty valuable stuff. Like Ishmael said, a feathering or folding prop gives you some offset aft which will give you more clearance. I ran it in a prop spec spreadsheet that works fairly reliably and came up with 14x9 3 blade with a Beta 25 2:1 at 6.5knots. That would be more than enough motor, you may want to go down to a 20hp 2:1 with a 13x8. Propping is still a bit of black magic so take my numbers only to prompt you to carefully measure clearances with different types of props. Horsepower is useless without a propeller to harness it.

#21 Becalmed

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Posted 14 November 2012 - 03:07 PM

Looks pretty close to that rudder post, better make sure a feathered or folded prop doesn't touch.

#22 Bob Perry

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Posted 14 November 2012 - 03:08 PM

Seth:
Why worry about the prop until you have chosen a new engine. We are going at this backwards.

#23 Anomaly2

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Posted 14 November 2012 - 03:13 PM

Seth:
Why worry about the prop until you have chosen a new engine. We are going at this backwards.


Hey, you're the one that started talking about props.... well, wait, not "you" Bob, but you guys.... (you Bob started talking about my bottom but I was just going to let that slide...). I just provided some prop info cause I thought it might be relevant to the engine hp choice if in fact I was constrained on the size of prop I could turn.... So, right, I'll focus on engines! And report back in if/when I have some candidates. (I'm left-handed: I do a lot of things backwards...)

#24 Bob Perry

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Posted 14 November 2012 - 03:52 PM

Seth:
Pick the engine and gearbox you want then we can work from there to look at prop options. If you call Max Prop with engine and gearbox specs they can give you very accurate prop specs and you can use these as a base. Then we can see with that prop spec if you have the tip clearance you need. If not we can then backtrack and find the right gearbox prop spec for your application. Keep in mind a three blade will requite less diameter than a two blade. I think Becalmed's data sounds about right. I'm not saying but max Porp, although I would, but they can provide you with specs and a rpice to get the process a little further along. Talk to Fred. He can do this stuff with his eyes closed. Or you can do the same thing with Kiwi prop and have some comparative data.

#25 SemiSalt

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Posted 14 November 2012 - 04:11 PM

For efficiency, you typically want the biggest possible prop rotating at the lowest possible rpm (without cavitating).


The blade design is also important. I get much better performance from my Gori folding prop than from the original fixed prop even though the diameter is the same. The sailing-type fixed prop has very narrow blades for low drag. The folder has wider blades and gives me a half-knot more at a given RPM.

#26 goob

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Posted 17 November 2012 - 03:15 AM

when I repowered with a Beta, I went with a Flex o Fold prop. The way the hub and blades are set up the blades moved aft on the shaft far enough to go with a larger blade by 2 inches.

#27 olaf hart

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Posted 17 November 2012 - 03:51 AM

So, I'm thinking about re-powering. Wait, make that I'm "considering" re-powering. Moe knows what I'm thinking about. Anyway, where was I? Oh yeah, so my old Perkins 4108 is showing some signs of wear and while I"m in no way committed to re-powering, I'm at least considering it in the sense of weighing up the tradeoffs between putting money into the Perkins vs taking the plunge. One big question I have is how much HP is enough? The boat is 33' LOA by 9'6" beam and displaces 11,000-12,000 lbs (that's just a WAG, Bob would certainly know better....). The Perkins is 50 HP and arguably way more than this boat needs. It certainly makes the boat squat at anchor more than I would like... I have a tiny prop (10") and no room to increase the diameter. The current gear is a 1:1 velvet drive. Lots of the smaller diesels I have looked at (20-25 HP range--- another WAG on my part) have been coupled to 2:1 and even 3:1 gears. which seems to be something else to factor in. But, I'm out of my knowledge range here. Any advice from this crowd would be appreciated.


You have seen our boat, about 11000lb loaded and about the same waterline length.
We have 250 hours on a new Yanmar 3ym30, including the NSW South Coast and Bass Strait.
Sweet motor, but its too much HP for the boat, we have never run it over 2400 rpm, and that is cavitating the prop at hull speed.
Normal cruise is around 2000 rpm.
When we did the commissioning run at 3200 rpm the stern dropped about two feet and the cockpit was awash.

The long term issue is the motor is never fully loaded, so it is likely to glaze up early.

Prop is a fixed 16" three blade, makes a good tugboat.

Previous motor was a Yanmar 2gm20, it was fine for the boat. I think it is better to go with a motor that can run flat out some of the time.

The triple is much quieter and smoother though, I think the Kubota Clones have a three cylinder motor in the 20hp range.

An earlier boat, around 28 feet with a 12" three blade in an aperture, couldn't use more than 10 HP.

#28 Elegua

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Posted 17 November 2012 - 03:59 AM

I wonder what Perkins 4-108 ever actually made 50hp? Maybe in the user manual.

When faced with the same choice, I rebuilt mine due to a v-drive installation. I like the fact that I can understand and work on most of the parts. I can remember as a kid working on a similar 4-108 in my parents boats. I like the low RPMs and If you sound proof the engine box with new materials like soundown and update the engine mounts, it really eases the NVH. So far, no oil leaks...

A 3-blade maxprop is a nice cruising choice, compromise of drag and thrust. It can be rebuilt for about a boat buck or less ad infinitum.

#29 Ishmael

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Posted 17 November 2012 - 05:04 AM

I wonder what Perkins 4-108 ever actually made 50hp? Maybe in the user manual.

When faced with the same choice, I rebuilt mine due to a v-drive installation. I like the fact that I can understand and work on most of the parts. I can remember as a kid working on a similar 4-108 in my parents boats. I like the low RPMs and If you sound proof the engine box with new materials like soundown and update the engine mounts, it really eases the NVH. So far, no oil leaks...

A 3-blade maxprop is a nice cruising choice, compromise of drag and thrust. It can be rebuilt for about a boat buck or less ad infinitum.


A friend was just told by PYI that his was no longer rebuildable due to too much wear, but they probably meant "affordably".

#30 Bob Perry

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Posted 17 November 2012 - 03:17 PM

I figure a healthy Perkins 4-108 is an honest 35 hp. To push the Esprit 37 at hull speed required 17 of those horses, according to the tach. That's way too much engine for Anom's boat.

#31 kimbottles

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Posted 17 November 2012 - 04:47 PM

I figure a healthy Perkins 4-108 is an honest 35 hp. To push the Esprit 37 at hull speed required 17 of those horses, according to the tach. That's way too much engine for Anom's boat.


For what it is worth we had a 24 HP Volvo Sail-drive in my old Swede 55 which displaced 18,000 pounds with a 40 foot waterline. 52x40x9'8" with 6 foot draft. She would motor all day long loafing at 8 knots and just under 1 GPH. She would top past 9 knots if we wanted to push her but the fuel consumption would climb towards 2 GPH. She had a 2 blade Maxi-prop (I think it was 16 inch diameter.)

For the Sliver design we have a 39 HP Yanmar Sail-drive. The Sliver will end up at a bit under 20,000 pounds displacement on a 55 foot waterline. 62x55x9'10" with a 10 foot draft. I expect she will loaf at 9 knots and push past 10 knots if we want to push her. (She will most likely have a 3 blade Gori of 18 inches diameter.)

Theory is great, but I always like to take into consideration the actual observed performance of vessels when it is available.

#32 stardate2010

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Posted 06 December 2012 - 08:57 PM

For what's it worth, my 4-108/50hp is connected to a 2:91 velvet drive, and at 3000 rpm burns just under a gallon an hour, and turns an 18x14 prop. PO said you can turn more rpm(4000) but you'd just be wasting diesel... this also is hull speed at that rpm... boat gross is 22 tons. Morgan 451




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