A question of math

[Dave Ullrich]

OK, I'm VERY bad at advanced math. I'd like my Cat 30 to cruise around 6.5 knots under power with power leftover to pound into a headwind. The boat weighs about 11,000 pounds. It has a waterline of 25 ft. This should provide a hull speed of about 6.7 knots, so my idea is possible with enough power. So how much power would I need? Right now I have the little 11 hp Universal 5411 and a 2 blade prop and it provides a cruise speed of about 5.5-5.7 knots and a top end around 6.0-6.3 knots. But from what I've seen here, others with a LOT more power don't go any faster than I. Is something wrong with their setup, or do you just need a huge power increase to create a small increase in speed?

 

[Ernie Rodriguez]

What the math shows..

For the numbers you mention, a waterline length of 25 ft and a displacement of 11000 lbs, 10 hp would give you a speed of 5.17 knots, 15 hp would be 5.91, 20 hp would give you 6.51 knots and 27 hp would be 7.19 knots. In my case, a Hunter 36, wateline length of 29 ft and a displacement of 14000 lbs, the numbers work out almost the same. 10 hp gives 5.13 knots, 15 hp gives 5.87, 20 hp gives 6.47 and 27 hp gives 7.15 knots.All of this assumes a propeller efficiency of 55% and one that is matched to the engine. A prop that is too "small" would not load ( not enuf torgue) the engine in order for it to deliver its rated hp at max rpm, and similarly a prop that is too "large" ( more torque than the engine can deliver) would not allow the engine to reach is max rpm, hence its max power capability. ErnieWith your two blade prop and your 11 hp engine, you are doing pretty good. A bigger engine would help to give you more reserve. A fixed three blade prop would give you a better overall performance characteristic at the expense of additional drag when under sail.

 

[Kevin L. Woody]

Question of drag.

Hello Dave:Your speed length ratio, or hull speed, is a number factored based upon a series of average denominators. The use of averages also falls true when you determine the Effective H.P. needed for a particular speed. These averages however become skewed when weight is added, Shaft H.P. becomes limited due to efficiency losses to the running gear, biological drag on the hull,… However the largest influence is the displacement of water by the hull. The greater the water to be displaced the greater the H.P. to accomplish this. At a point however of displacing water with the hull, the amount of fluids being displaced exceeds the hull shape thus creating a wake or bow wave. As the bow wave increases so does the amount energy required to push the hull up and over the increasing height of the bow wave. At the point which the hull pops up and over the wake is the point your Catalina 30 has transformed itself into a 30 foot Boston Whaler or planing hull. The displacement of those fluids efficiently at the bow becomes the unknown, which requires us to use generic hull speed factors. ( Square root x 1.35 water line) If the bow was a plum bow but was four feet across and two feet deep your tapered raked bow would have an easier time of displacing water. One can most likely expect that you will not have the ability of motoring above or even motor at the projected hull speed even if you increase the H.P. Of coarse this is within reason. Possibly it would require not 10 H.P. like know but 50 H.P. to achieve the small added speed. But increasing H.P. will result in just pushing a greater amount of water at an extreme cost to the boats current efficiency. Added weight and added fuel cost to name two. Hope this helps. Kevin L. Woody

 

[Neil@mccubbin.ca]

Try a 3 blade prop

I had a 25 HP motor in a 40 foot, 12 ton boat a number of years ago. 15" pitch 2 blade prop.She did about 6 knots in easy conditions, but dropped off miserably, below 3 knots, in a chop/headwind.Changed to 13" pitch, 3 blade, and kept the 6 knots in normal conditions, but held 5 in bad conditions. Also power astern to stop fast, and steering going astern, improved dramatically.I woue at elast change to 3 blade, err on the low side fo the standard calculations for pitch. By far the best answer is a 3 blade Max prop, then you get all the benfits of self feathering, and can adjust the pitch to the optimum. Expensive, but easier and cheaper than a new motor