Horses on a 20' center console

I have what is considered a light planing hull, I am told, for a 20 foot center console. (A seacraft "variable deadrise hull") It is 1600 lb. With a modern outboard, fuel and passenger and a bit of gear, it is probably 2400lb. It will do just under 30 knots with 100 hp. it is said that 70hp is the minimum for this hull. Some install more than 200 hp on them. I can't see why.

 

The boat in Phils post looks like a Wolverine, Yellow Jacket, or similar molded hull with a Johnson 25. Those boats with that motor would run about 25 MPH with two normal sized passengers. All up weight including the two passengers, would have been in the 750 pound range for a 16 footer similar to the picture. Hard chine boats of similar size and weight were only marginally faster.

The boats of the that era, the 50s and early 60s, were considered entirely satisfactoy for most purposes. Gradually macho madness crept into our culture and rendered sensible boat rigs acceptable only for geriatrics or wusses. For some reason we now need 60 or 70 mph rocket boats in order to prove our manliness as in; "Mine is bigger than yours".

 

Thanks for all the constructive info everyone.

PAR, I've got two versions it drawn up- one on paper one in CAD and I don't have either on the laptop with me this weekend. You are correct about my weight numbers, the 620 was just the ply hull and my initial estimates for this boat - actually 19'-2" LOA and 17'-10" LWL (idling) - is closer to 1200 before the men and beer. Considering the guy that I'm doing this for though, with him and his brother together on the boat it may be well over the 1800lbs mark. If it was me and mine, it would be about 1500 lol. Opposing ends of the proportional spectrum for sure, that I have to account for...

Running through the replies though, I've got the info and answers I was looking for, mainly the discrepancy between the production boats and the calc outcomes. I'll try to get some pics up after the weekend though, at least some lines.

 

We have a difference of opinion here, which is usually normal for the forum. Here is another boat a bit larger than the other one I posted and which I am familiar with. http://www.bandbyachtdesigns.com/ocracoke.htm

I can verify the 42mph speed with a Honda 90 and five adults aboard because I was aboard checking the speed with a GPS. As you can see from the build photos, it is not really lightly built and is covered with 1808 Knitex glass, inside and out for a tough long lived package. The hull performs well in rough water which we have an abundance of on the NC coast. It's rated to 150hp because some people demand it but, in my opinion, it just ain't necessary. 90hp gives plenty of reserve power for any reasonable use I can think of.

I know some will disagree with my thoughts on power as inadequate here but the water is big enough to accommodate all opinions.

 

I would think a realistic cruise speed of 20 knots would be about the mark for that 13-degree boat in normal usage, which would equate to about 90hp using the formula I posted earlier. ( I have assumed waterline of 5 metres and WL beam 2 metres) The 9" (.229m )draft was kindly provided !

 

I'm puzzled. I agree that 20kts is a realistic cruise speed for this boat but it says nothing about potential.

Formulae are just formulas. The results I gave are factual.

 

Using a Crouch constant of 180, 150 hp should get you 52 mph speed with 1800 pound displacement. That is faster than I imagined you were planning on. The Crouch constant I use came from a 18 1/2 foot lightly-built planning hull I completed last year. That boat gets 33 mph with 50 hp motor (at 7000' altitude) and two people aboard. With 75 hp and 1500 pound displacement, top speed for your boat should be about 40 mph. Choose your power and your speed, but the trade off will be in fuel economy.

 

This already exists ...

Two highly efficient center console boats that meet the original poster's performance criteria exist and are available in kit for today. Russell Brown/Paul Beiker's PT Skiff and Graham Byrnes' Marissa. Both were designed for Woodenboat Magazine's eco-challenge contest. I'd build either one in a second.

Both are well documented, peer reviewed and do exactly what is wanted - over 20 knots with 20 horsepower or less, and great fuel economy as well - in functional center console configuration.

--
CutOnce

 

Those boats, particularly the second one, would bash you mercilessly in anything worse than a small chop, at 20 knots. The first boat looks pretty tippy to me, as well. Those types of craft are suited to protected waterways only, which maybe the OP has in mind anyway. Another point is that it is only useful to talk about what speed a certain hp engine will attain if there is still plenty of throttle left, running at peak rpm will send the fuel flow skyward, and be less efficient than running a bigger motor.

 

I agree with your analysis of the two boat hulls; however, I am not so sure about fuel efficiency in a modern outboard. With the new ETEC 50 hp installed on my boat, so far I average almost one gallon per hour in fuel consumption. I don't always have the throttle pegged, but I certainly don't worry about fuel consumption at any speed. I think that with modern electronic engine management, the efficiency is good throughout the speed range. Perhaps the engine is being stressed more, but it is running within the recommended RPM range, 5500-6000.

A larger engine means higher initial cost, more weight, and heavier scantlings for the hull. Forget "lightly built".

 

Even with all the modern features of today's outboards, fuel flow goes to the devil at WOT, even the manufacturer's tests charts show that.

 

I take issue with the implication that a small motor producing X hp will use more fuel than a larger motor producing the same X hp even though its throttle is set lower. Engines efficiencies are measured by a test of brake specific fuel consumption (BSFC). That is the number of pounds of fuel an engine consumes for each horsepower in one hour. BSFC numbers like 0.45 to 0.50 are most commonly seen in dynomometer tests of gasoline engines. When a specified horsepower is given the BSFC number will give a very close estimate of fuel consumption without regard to piston displacement. Small variations can be seen on a plot of different RPM but does not by any means skyrocket.

The only likely difference in efficiency is that a larger motor may have more prop area which might have less slip than the smaller motors prop. That point I cheerfully concede.

 

Running an engine at continuous peak revs isn't going to be ideal to make it last. Just looking at some info from suzuki 4-strokes tests, and a lightweight alloy boat fitted with a 40hp engine got .45 litres/NM at peak revs, while a somewhat heavier plate alloy boat equipped with a 70hp from the same maker gets the same figures at a backed-off 4500rpm. A more capable boat getting the same mileage. Same speed (44km/hr) from both boats, one 400kg the other 800kg. The heavier boat with the 70 has to be a better idea than the overworked 40.

 

I agree that the right answer is to check the BSFC map and up to a point, reduced RPM is a good thing for both wear and economy. But low loads reduce efficiency, so you need more pitch on the prop to keep the engine loaded. The common "requirement" to achieve xxxx RPM at WOT is not correct from a wear and economy standpoint.

At some point, low piston speeds also reduce efficiency.

 

All small boats bash you mercilessly if you run faster than their intended range in big chop. The deep V is no exception as the cocoon padding in offshore racers shows. Marissa runs very well in significant chop, especially if you keep the bow down and let that sharp entry part the waves. PT Skiff will too and the tippy claim is true but, all boats are compromises in one way or another. The OP's goals would be satisfied with either if he accepts their particular limits. Either needs far less power for reasonable speed than many suggest.