What hp to re-power a 38ft sailboat?

[malbert73]

Originally Posted by Bowdrie

My 35ft boat, LWL ~28.5ft, and fairly well loaded is at ~18,500>19,000.

It has a lot of wetted surface with its cut-away full keel/attached rudder.

The engine is an old 4cyl. Alaska Diesel Lugger, a long stroke industrial block that with 130cu.in. only turns 2,500rpm for 40hp.

I used a Michigan Wheel 3-blade, (lots of blade area,) 18x14 and made a trip North from SoCal to Astoria, wind and sea on the nose most all the way/most all the time.

Typical cruising was 1450, (an easy on the noise and nerves sweet spot,) for ~5>5.5kt average speed.

The prop was overpitched, the engine would only get to ~2,000>2,050.

Had the prop re-pitched to 18x12, (recommended spec,) and now it will just nudge the governor.

The Hurth box is 1:88 reduction.

Although max available thrust is up, the engine now needs to turn 1600 to get the same boat speed that 1450 would do before.

I think the ideal would have been a 18x13, a sacrifice of a few top-end HP would give better loading in "normal" operation, but I will say that in close-quarters maneuvering the response is "right now", no waiting.

For the OP, rebuild the Perkins, save money for more beer and other essentials.

Lugger- great engine!
Yes that 18x13 prop probably would have been the ideal overpropping for real world cruising

As for the 4-107, if only it extracted the 40hp at 2500 like the Lugger. But it takes 3000RPM + to get the HP out of it.
Get a Beta!

[merrydolphin]

Originally Posted by Bowdrie

My 35ft boat, LWL ~28.5ft, and fairly well loaded is at ~18,500>19,000.
It has a lot of wetted surface with its cut-away full keel/attached rudder.
The engine is an old 4cyl. Alaska Diesel Lugger, a long stroke industrial block that with 130cu.in. only turns 2,500rpm for 40hp.
I used a Michigan Wheel 3-blade, (lots of blade area,) 18x14 and made a trip North from SoCal to Astoria, wind and sea on the nose most all the way/most all the time.
Typical cruising was 1450, (an easy on the noise and nerves sweet spot,) for ~5>5.5kt average speed.
The prop was overpitched, the engine would only get to ~2,000>2,050.
Had the prop re-pitched to 18x12, (recommended spec,) and now it will just nudge the governor.
The Hurth box is 1:88 reduction.
Although max available thrust is up, the engine now needs to turn 1600 to get the same boat speed that 1450 would do before.
I think the ideal would have been a 18x13, a sacrifice of a few top-end HP would give better loading in "normal" operation, but I will say that in close-quarters maneuvering the response is "right now", no waiting.
For the OP, rebuild the Perkins, save money for more beer and other essentials.

Great engine. But again, 1:88 reduction does not give you anywhere near what a 2.5 or so at least, would give you the ability to swing a much bigger prop and have even more power. (If there is room to swing a larger prop). It’s like trying to go up a hill in high gear. Boats under power are analogous to constantly climbing a hill and never getting to the top. Cars/ trucks reach a coasting or rolling speed once done accelerating. Boats do not because of the density of water and the resistance rapidly increasing with speed. My Cal 2-46 had a 2.9 to 1 and swung a 26” prop! It could moter as well as a trawler but could still sail 200 mile days. It was a heavy cruiser of 34,000 lbs. and an old but very good Perkins 4-236 85 hp. Great engine. That Luger another good one! But 1.88 is almost a punishment. But like so many boats you might be limited by prop clearance space. If so not much you can do. This is a dilemma of the design of auxiliary sailboat power. Experienced cruisers prove that unless you are in a trade wind route, it is not unusual to motor 50% of the time.

Smaller props are fine until you get headed with wind and seas and then slippage enters in. Many Aux sailing boats can nearly stop in these conditions. The only way to make headway is to motor tack with main up. Have had to do it for days. With the Cal, put the pedal down take all sail down and steer dead up wind with good speed. Even against strong wind and seas. 8-10 and 25+ kts wind and I could hit near hull speed.

[Bowdrie]

Originally Posted by merrydolphin

Smaller props are fine until you get headed with wind and seas and then slippage enters in.

Yes, we all understand what you are saying, but for boats in the size range the OP is talking about, an 18in. prop with ~35>40HP is pretty much/more-or-less the standard, and few boats in that size can swing a prop much bigger.
Comparing to a 46ft boat is an apples-to-oranges thing which has no application to the OPs inquires.
As with anything else on a boat, one does what one can to achieve the best results that are reasonably attainable.
You do the best you can with what you've got to work with and accept that all else is merely conjecture.

[toerail]

Electric, if your a DIY and secure with it

[Laxlife]

40hp minimum

[Uncle Bob]

Originally Posted by Laxlife

40hp minimum

+1. 50hp kubota based would be my choice.

[FPNC]

40 hp minimum with an 18" diameter, 3 blade prop preferred. A 17" dia, 3 blade as a minimum. The 4.107 was realistically a 36 hp engine.

[cottonsail]

2.5 HP per thousand pounds of displacement. Just that simple.

[Adelie]

Originally Posted by cottonsail

2.5 HP per thousand pounds of displacement. Just that simple.

More power creep.

Long ago when motors and fuel were expensive and heavy the rule of thumb was 2hp-ton. That could get you to hull speed in calm conditions and to several knots heading into 20kt wind with significant fetch.

Then motors got lighter, cheaper and more efficient and fuel came down in price so the recommendations creeped up to 3 then 4hp/ton. And you can go hull speed heading into 20+kt with significant fetch.

Now we’re starting to see 5hp/ton (2.5hp/1,000lb).

[FPNC]

Originally Posted by cottonsail

2.5 HP per thousand pounds of displacement. Just that simple.

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.

[AJA]

I have a 37’ , 18000lbs displacement. I just repowered from a 40 hp Volvo Penta to a 30hp Beta. Couldn’t be happier. The 30hp Beta gives me better performance than the old Volvo.
No problem moving through a 30kt headwind and chop.
Now I should also mention that you’re prop choice is critical.
My dealer advised me that I was using the wrong prop and had me consult a prop guy to find the best prop for the specifications of my transmission and hull. Still I can’t say enough about Beta. During installation, I had some questions Called the guys in the UK, and they were extremely helpful. Made feel supported all the way throughout the process. The help line goes directly to the techs. No answering machine or front office involved.

[Gripfast]

Originally Posted by AJA

I have a 37’ , 18000lbs displacement. I just repowered from a 40 hp Volvo Penta to a 30hp Beta. Couldn’t be happier. The 30hp Beta gives me better performance than the old Volvo.
No problem moving through a 30kt headwind and chop.
Now I should also mention that you’re prop choice is critical.
My dealer advised me that I was using the wrong prop and had me consult a prop guy to find the best prop for the specifications of my transmission and hull. Still I can’t say enough about Beta. During installation, I had some questions Called the guys in the UK, and they were extremely helpful. Made feel supported all the way throughout the process. The help line goes directly to the techs. No answering machine or front office involved.

When did you order the engine? Been waiting for my Beta 30 Since early Janaury. Not suposed to be in the US until late May.

Also, what is the specs on the prop you bought?

[team karst]

Originally Posted by cottonsail

2.5 HP per thousand pounds of displacement. Just that simple.

Unless its a sailcat.
I think 8 HP/ ton is more typical for those.

[malbert73]

Originally Posted by team karst

Unless its a sailcat.
I think 8 HP/ ton is more typical for those.

Yup- that’s the proof that cruising cats are not necessarily faster than monohulls except in really big breeze…

[skenn_ie]

Depends on whether it is, or you want, a motor sailer or just a docking engine. I frequently use propking. It agreed with another tool that people here kindly suggested, but gives a few more options.