Speed Vs Hp?

[Dave Ullrich]

I know this sounds wierd, but I have a Universal 5411 that produces 11 hp. I'm using the standard 2 blad prop that came with the boat. What I find interesting is that I can achieve the same speeds as other are getting with 18-20 hp with my "little engine that could". I cruise at 5.5-5.7 and top out near 6.3 knots. Humm... OK, what's up with you all that are having problems with 2x as much power? Any comments on this would be greatly appriciated.  I was thinking of repowering with a bigger engine, but from what I've heard here, stepping up to 20 hp really won't get me any more speed. What I really want is to be able to cruise under power near hull speed and have something left over for powering into a strong headwind and still keep that turn of speed. So what's that going to take? It seems like going to 20 won't be enough. Would I need 30? 40? At that point it become pointless, as those engines would never fit in a Cat 30 and right now the engine I have sips less than 1/4 gal per hour at cruise speed. So is my thought of cruising at near hull speed a "pie in the sky?"

 

[Ernie Rodriguez]

If you like math...

Dave, if you want to go through some math excercises, use Dave Gerr's ("Propeller Handbook) equation for determining power required for a given speed: SL RATIO = 10.665 / cube root of (LB/SHP)  where SL RATIO = Speed length ratio = Kts. / Sq Root of WL (water line length), kts in knots and WL in feetLB displacement in pounds, and SHP shaft horsepower at propeller. About the best to expect will be a SL of about 1.34 if you have enuf horsepower, but typically you will find 1.2, which is probably what your boat is sized for. A bigger engine might give you 1.34.Hope this helps, and have fun.Ernie

 

[Richard Gresham]

Teck talk

Dave, If you have the tech talk collection there is one article that sounds like your story. I have a universal M-25 in my 87' C30 TR and at about 2100 rpm I can do 6 knts with a 13 x14 prop. Others say they can get 7knts with the same setup at 3500 rpm but, I haven't pressed my luck at those revs. If you want more power for going into the wind and chop then you'll want more HP and a different prop. Just changing your prop pitch won't do it....good luckRichard

 

[Kevin L. Woody]

Speed relationship

Dear Dave:The problem is Dave, that you do not have the same bag of old bones for sizing like us propeller guys. The relationship between speed and HP do not always fall consistent between differing hulls and power plants. Your particular 30 foot boat may have a 9.89 Effective H.P. required to drive her at the 6.3 max knots. Whereas a 34 foot boat may have an Effective H.P. of 14.5 to achieve the same speed. In all cases the resistance of the vessel moving through the water is one of the main determining factor's in determining the required H.P. There is a great book, which I use, along with Dave Gerr’s book, which is entitled Skenes Elements of Yacht Design. Far more info goes into this kind of a discussion then I have listed. Having a bag of bones will also help. Hope this helps. Kevin L. Woody

 

[Neil@mccubbin.ca]

Prop efficiency is probably the key

I suspect you have a prop that matches your boat well. Most are mismatched, with the reault that efficiency is poor. No prop is very efficient, but a mediocre selection can put half as much useful power into the water as a perfectly matched one.The standard selection techniques and practices seem to be to lead to overly high pitch forr many, if not mst, sailboats.