What hp to re-power a 38ft sailboat?

[cottonsail]

19000 (your boat) X 2.5 (HP per thousand lbs) = 47.5. That's your bottom line for HP. Our boat is 16000 lbs and we went with a D2-50F Volvo Penta. We couldn't be happier now that it's done. The engine is great but dealing with Volvo Penta USA is just horrible. The new crated engine does not come with an installation manual. That can only be downloaded by a Volvo tech from a secure web site. Bought the boat with a seized Volvo MD2B. Replaced that with a rebuilt W40 Westerbeke. Leaked oil and constant small problems. Finally went to a brand-new Volvo D2-50F. The Westerbeke was good for 6.2 KTSat 1 gal per hour. The new Volvo is doing 6.9/7 KTS at 1 GPH. If I had to do it all over again, I would have gone Beta for the customer support. Never again Volvo Penta.

[wingssail]

Originally Posted by FPNC

4 hp per long ton will be very adequate and will easily push most every sailboat to hull speed in calm water. Provided the propeller is not some wonky size. In a head sea or wind you may be held back a bit.

4.5 Hp per long ton is more that adequate for most every sailboat that is not a motor sailor. This will provide all the power you need in most any conditions. With the correct propeller you will not lack for power.

However the prop is an extremely important factor and cannot be overlooked when sizing an engine. Match the HP to the propeller that can be fit or you will wind up with an inefficient system. More HP can actually make the vessel slower if the propeller is not properly sized to transfer that power to the water in an efficient manner.

There are some ~19 long ton sailing vessels on the water being pushed quite nicely with 3 hp per long ton. It works well because they can fit a good adequately sized propeller that increases efficiency.

4-5 hp per long ton? These recommendations seem better suited to motor sailors than sailboats. What kind of boats do you folks have anyhow?

We put a 34hp Yanmar into our 43' 18000lb sloop. (that is 1.8 hp per 1000lb displ, 4 hp per long ton). We can hit hull speed of 7.2 knots when the bottom is clean. We can motor into 40 knots at more than 6 knots if the water is flat, and that is with a 2 blade Martec folder (18x18). But we don't. If the wind is blowing we sail.

You'all should try it.

[ChrisJHC]

I’ve also heard a “rule of thumb” of 1HP per foot of boat length.
e.g. a 30 footer with a 30HP engine.

Sounds in the ball park.

[FPNC]

What model vessel?
What is the waterline length?
Displacement?
Maximum Propeller diameter that will fit?
Type of transmission used?
Original (or current) engine?

These are the details you need to provide in order to get a correct answer to your question.

[Hoodsail]

Originally Posted by wingssail

4-5 hp per long ton? These recommendations seem better suited to motor sailors than sailboats. What kind of boats do you folks have anyhow?

We put a 34hp Yanmar into our 43' 18000lb sloop. (that is 1.8 hp per 1000lb displ, 4 hp per long ton). We can hit hull speed of 7.2 knots when the bottom is clean. We can motor into 40 knots at more than 6 knots if the water is flat, and that is with a 2 blade Martec folder (18x18). But we don't. If the wind is blowing we sail.

You'all should try it.

Wing, we have a 25hp Westerbeke on our 35' (27 LWL), 15000lb sloop...I never hit hull speed of 7k even when the bottom is clean (1st month after spring launch). We can do > 5k in calm seas, even into the wind...do 3.5-4k in 3-5 ft seas. Only did 3-3.5 k coming back from Bermuda this last June, but that was into 3-5 footers and about 15-20k on the nose...heading to Montauk. That's with a 14inch, 4 blade feathering prop...wished we had 10 more hp then!! And yes we could have sailed but choose not to tack across the north atlantic!

[Jim Cate]

Originally Posted by Hoodsail

Wing, we have a 25hp Westerbeke on our 35' (27 LWL), 15000lb sloop...I never hit hull speed of 7k even when the bottom is clean (1st month after spring launch). We can do > 5k in calm seas, even into the wind...do 3.5-4k in 3-5 ft seas. Only did 3-3.5 k coming back from Bermuda this last June, but that was into 3-5 footers and about 15-20k on the nose...heading to Montauk. That's with a 14inch, 4 blade feathering prop...wished we had 10 more hp then!! And yes we could have sailed but choose not to tack across the north atlantic!

I suspect that your poor motoring performance is a factor of the tiny feathering prop more than a lack of horsepower. The combination of small diameter, large hub and flat blades (no twist) is not a happy one!

Jim

[MWGDVC]

David 97,
My boat has similar specifications to yours, and is equipped with a healthy 4.108. I realize this is anecdotal, but I make 5.5 KTS. at 1400 RPM., make a little over 7 KTS. at 1750 RPM. with a clean bottom. I averaged 0.6 gallons per hour over a 600 mile motor up the Tenn. Tom. waterway. I would rebuild the Perkins before I would replace with another powerplant. I am satisfied with the performance of the 4.108.

[wingssail]

Originally Posted by Hoodsail

Wing, we have a 25hp Westerbeke on our 35' (27 LWL), 15000lb sloop...I never hit hull speed of 7k even when the bottom is clean (1st month after spring launch). We can do > 5k in calm seas, even into the wind...do 3.5-4k in 3-5 ft seas. Only did 3-3.5 k coming back from Bermuda this last June, but that was into 3-5 footers and about 15-20k on the nose...heading to Montauk. That's with a 14inch, 4 blade feathering prop...wished we had 10 more hp then!! And yes we could have sailed but choose not to tack across the north atlantic!

Well, your waterline is 7ft shorter than mine so your normal top motoring speed would not be 7kts like mine is. Based on my calculations you should see 6 kts.

Like Jim said, probably your lower speed than that, especially into headwinds and waves, is due to a small prop. And 25hp is a bit light to my way of thinking.

Chose not to tack across the North Atlantic? We would have. Heading into waves and a wind on the nose, you have to beat. That is sailing. You know it's going to take longer, you just bite the bullet and do it. We do that, every time. Some times you hate it, but we hate pounding int those waves under power even worse. Motoring may seem quicker but it's no fun, to my way of thinking.

[cottonsail]

Originally Posted by ChrisJHC

I’ve also heard a “rule of thumb” of 1HP per foot of boat length.
e.g. a 30 footer with a 30HP engine.

Sounds in the ball park.

That was in the 1970's and '80's. Today it's 2.5 HP per thousand.

[skenn_ie]

Feathering or VP props are always a compromise. Feathering is a compromise between speed under sail and speed under power, usually with a heavy bias towards sailing. VP props are optimised for a particular speed/load. In other modes, the twist isn't right.

[NYSail]

My 19,500lb boat has a 54hp engine and runs beautifully. Good advise above…..

Good luck
Greg

[FPNC]

Originally Posted by wingssail

4-5 hp per long ton? These recommendations seem better suited to motor sailors than sailboats. What kind of boats do you folks have anyhow?

We put a 34hp Yanmar into our 43' 18000lb sloop. (that is 1.8 hp per 1000lb displ, 4 hp per long ton). We can hit hull speed of 7.2 knots when the bottom is clean. We can motor into 40 knots at more than 6 knots if the water is flat, and that is with a 2 blade Martec folder (18x18). But we don't. If the wind is blowing we sail.

You'all should try it.

Okay, let's try that.

18,000 lb / 2240 = 8.03 long tons

34 hp / 8.03 = 4.23 hp per long ton.

Sounds about right to me.

[team karst]

Originally Posted by cottonsail

That was in the 1970's and '80's. Today it's 2.5 HP per thousand.

in 30 more years, it might be .5HP/thou

[AKA-None]

Originally Posted by team karst

in 30 more years, it might be .5HP/thou

And they’ll be old planning hulls at that time

[MicHughV]

If you truly want to dive into this....

Get the "Propellor Handbook" authored by Dave Gerr.

It's a very technical read, and may require some serious attention from your brain cells, but when you're done, you'll know everything you wanted to know. The book details methods to figure this out yourself. Short of that, as recommended above, something in the 30-40 hp range ought to do the trick, depending on gear reduction, etc.

Finally, to get the best of both worlds, you'll need a feathering prop. Yes, they are expensive, but worth every dime. Being familiar with Maxprop, I would never sail with a fixed prop ever again. There are, off course, also other brands available.