Minimum power to plane 22

I agree there are pros and cons for each system. I didn't pick twins; I bought a used boat with twins already on it. After learning that I can't plane on a single 50 a lot of the mystique is gone. If I were doing it over I might opt for a large single and kicker, I definitely see the benefits of one engine in the water at a time and separate systems. High on my list of dislikes for twins is the cost of maintenance and up keep, ouch. Fortunately I have separate electrical systems.
 
professional hobbiest-

I believe the formula for displacement boat speed is one derived from experience and observations, not a purely mathematical, vector, or lab derived analysis.

The "constant" factor of 1.33 or so, is for full displacement hulls, and a fundamental starting point, but different for other types of hulls, such as a flat dory or semi-dory hull. However, it is impractical to derive a special number for each of the possible hull types, which are infinitely variable.

You're right that the larger the power that is applied at displacement speed results in some gain, but the boat is pretty much locked in that displacement mode until sufficient power is applied to make it climb up onto it's bow wave and plane.

I don't think we're really diagreeing much here, you're just taking rightful exception to the idea that some abstract mathematical formula is an iron-clad predictor and limiter of boat performance. We all know that's not true, and the formula is just a derived predictor that is only so accurate.

Incidently, small gains in displacement speed with relatively large increases in power are the stuff of which large keel sailboat races are made of, although many of the modern high-performance boats would seem to reach planning speeds when surfing down large ocean swells.

Cheers!

Joe.
 
Somewhat off topic, but the previous message brought back memories -

About 20 years ago I was crewing on an ultralight Santa Cruz 27 in the S.F. Bay area. We were relative newcomers, but learned a lot over a few years. One race went out the Gate, around the Farrallon Islands, and back (25 miles out?). On the way back we were on a run/broad reach with the spinnaker, and surfed/planed for about 15 of those miles. Wahoooo!

BTW - ever tried sailing in the Gate on an ebbing tide? Not the best idea . . . (Boy, those structural foundations are big!)

Ed
 
Not to muddy the waters but I get about 14 mph at 4500 rpm (WOT) with one 40 HP Honda in the water and I'm on plane. That's with a day trip load (2 people + 30 gal gas + 10 gal water) on board and a 12 inch pitch 4 bladed Solas prop. I may have the RPM wrong, it may be as low as 4200, but I remember the 14 mph when I did it and thought that was not too bad. For the record, I get 5300 (WOT) and 28 mph with both engines operating with the same load. I plan on trying a 10 or 11 inch 4 bladed prop next summer because when loaded for a week cruise with the family of 4 I only get 4900 or 5000 rpm at wot. Yes, a week with 4 on board is a challenge but luckily we have a companion boat that's 27' with only 3 on board. BTW average mileage for the whole summer (approximately 800 miles each summer for the last 4 summers) has been consistently 4.5 to 4.8 mpg (that's statue miles).
 
Hi Pro Hobbyist

Carrying an extra prop is good. Changing the prop underway can be a real challenge. I would get the pitch set before I left dock and carry a spare prop of the same pitch. The single will get you back. I don't like hanging off the transom messing with the props. My problem with this is what happens when something or someone falls overboard. You could be in the water (worst) or your needed parts on the bottom (less worse)
and dead in the water (kind of embarrassing). ie Ahh thanks for towing me back after I dropped my cotter pin instead of I am glad I have two engines and one brought me back.

Safe boating

1tuberider
Jeff
 
1TUBERIDER":3um9ikuk said:
Hi Pro Hobbyist

Carrying an extra prop is good. Changing the prop underway can be a real challenge.

1tuberider
Jeff
Click to expand...

Jeff

Sure know what you mean about the challenge. Don't think I could change one from the transom. We use the propulse with the replaceable blades, which are a little more difficult to change than the complete prop. In the middle of Fredrick Sound, Alaska Inland Passage we hit something and dinged up two blades bad enough they had to be replaced. Used our Mokai to access and replace and it was far from easy. With any larger swell or chop would have been impossible. With duel motors we could have continued on with one, but being as the wave conditions weren't to bad decided to make the attempt.

Jay
 
With a single main and kicker, there is room on the transom for a swim step. Rough weather could still be a significant problem, but if calm, it is relatively easy to stand on the swim step and change a prop. Non-skid surface on the step makes this more comfortable!

John Frazey might be able to comment further on this option.

iggy
 
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