How much power?

[Doug B]

I'm looking at a 21' cedar on oak inboard hull that is similar to a maine lobster boat; round chine, skeg built. How does one determine how much power it takes to drive a hull like this (semi-planing?) to a certain speed. Currently the engine is a defunct 55hp ford diesel, that according to the previous owner would only give 10 knts. If there isn't a rule of thumb perhaps someone could recomend a book on the subject.

[dredbob]

_The Propeller Book_ by Dave Gerr would be a good place to start. You need to know the displacement (weight) of the boat.

Bob

[David G]

My local Yanmar dealer has software that inputs variables such as hull weight, LWL, etc. to answer such questions. I'd suspect other engine suppliers do as well.

"To accomplish great things, we must dream as well as act" -- Anatole France

[paladin]

I would expect 25-30 mph with that engine and craft if the prop is any good?

[mmd]

I expect that you are being overly optimistic, Chuck. A round-chined, skeg-built hull isn't the most efficient planing hull around so to get performance better than about S/L=2.5 requires gobs of power, relatively speaking. Cedar oon oak isn't going to make it a particularly lightweight, either. There is also the issue of controlability; Downeast and Cape Island type hulls become increasingly squirrelly to handle when driven beyond a certain point.

A typical Cape boat of that size (I am more familiar with these than Maine boats) with a 55-hp inboard will max out at maybe 15 - 16 knots. To hit 20 knots would take about 80hp, and then you start talking some serious weight if you stay with a diesel.

Not meaning to insult anybody, but if you want to go fast in a small boat, get a speedboat; seakeeping and carrying capacity in small boats travels a little slower.

[Ian McColgin]

Glad you chimed in Michael. Squirrely !?!? Ever see the pix in the National Fisherman of lobsterboat races where one really takes a tumble? You can add power and lifting strakes and all that - and I can see the end of season fun in taking the working mill out, putting in some hog, strapping on a PFD and the old HS football helmet and charging - but it's fun like unsanctioned stock car racing and just about as dangerous.

[Doug B]

Thank you for the responses. I had fogotten about Gerr's book, probably because it gave me fits trying to size the prop for my catboat. I'll try again. I don't really want this boat to go fast. I'm just curious about its capabilities and don't have mush experience with power boats. I did learn that the boat originally had a 4 cyl. chevy of 140hp. and that it might have done 20 wide open. Time to start looking for a small diesel.

[Ian McColgin]

I'd be intrigued with Michael's comments but so long as you're thinking of repowering, why not go with a controllable pitch prop?

[paladin]

Probably too optimistic.....There's a fellow at Herrington Harbor with a 25 foot Chesapeake Sharpie hull made of ply on frame, engine mounted just forward of the transon with V drive that gets 15-16 mph with less than 30 hp...the engine is rated at 28hp...the boat originally had a 60 hp perkins? and did about 20 mph...flat bottom with central skeg....boat pounds....I agreed to take the lines off the boat when I feel a bit better and can walk without groaning.....

[Doug B]

Ian
I've always thought a















Ian
I've always thought CP props were a great idea, but I'm looking for 15 to 16 kts. and am under the impression that they are not effective at higher speeds. The bottom line here though is the bottom line. If this happens it will be on a budget so the repower will most likely be a "running take-out". A controllable pitch prop would be a bit too pricy I think. The other thing is in all my years on the water I've never seen one.

[Spokaloo]

Just throwing out that not only is the Propeller Handbook a good resource, but so is The Nature of Boats, same author.

E

[Pierce Nichols]

The Propeller Handbook is on my shopping list for when things calm down a bit on the home front. FWIW, Dave Gerr is also readily accessible by email. I emailed him a couple of question on his book Boat Strength and he got back to me in a couple of days. A readily-accessible and friendly author is a great extra for a technical book to come with.

[mmd]

Which of my comments has intrigued you, Ian?

[Ian McColgin]

Comments yet to be made about controlable pitch props, which for some reason are far less common on this side of the Atlantic.

[mmd]

I think CP props are great... except.

Except the ones you have to haul or dive to change are too much bother (get it almost right the first time, then haul & tweak for final pitch, OK?) and operator-remote-controlled ones are too spendy for small boats.

But that's just my opinion...