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Catalina/Capri 25/250 Sailor's Forums

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how much horsepower?

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JimB517
Past Commodore

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USA
3285 Posts

Initially Posted - 06/16/2003 : 13:26:43

I was remembering a visit to the US Constitution some years ago. One of the deckhands told me that the sails generated 600 HP. It made me think, how much horsepower do the C25 sails generate? I know that about a 8 HP engine will move the boat at hull speed in calm water, and that props are about 25% efficent. So maybe putting 3 HP into the water moves the boat at hull speed. But when a 150 genoa and main sail are full on a beam reach in 10 knots of wind the boat seems so much more "alive" and powerful than when under engine power.

Do you think the sails are generating more hp or is the "alive" feeling because the center of effort has moved forward and the keel and rudder are generating lift?

<img src="http://www.indiscipline.org/cat25/pictures/icon.jpg" border=0>Indiscipline 1978 FK #398

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Arlyn Stewart
Master Marine Consultant

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USA
2984 Posts

Response Posted - 06/16/2003 : 21:40:48

Jim, I have exceeded hull speed many times while under sail...but never while motoring. In fact, with the Honda 8, full speed is reached at about 2/3 throttle... more throttle than that, and I start to lose speed. My theory is that exceeding hull speed requires significant more power and no drag. In fact, the last knot of speed takes the 2nd third of throttle. But once the prop looses bite and starts cavitating a little, then its efficiency is lost. Thats the primary reason I didn't go for a 9.9 over the 8... if cavitation begins at some point within the rpms of the 8 hp... then the extra 500-1000 rpms is simply not needed. Another thing to think about is that even though the prop is pulling, the lower end is in the water offering a lot of drag.

I once enjoyed a full 2 hrs making 7.3 knots which of course exceeds the hull speed of the c250 being slightly less than the C25. The helm was locked and the boat needed no attention. This occurred on a close reach with about 17 knot winds and less than a meter chop with full main and two turns furled on the 110 jib. When winds increased, I took the first reef on the main... and dropped to hull speed. I tried full jib and every combination possible but could not break hull speed and get back to the previous speed while under reefed main.

The experience of course verified the debunking that a lapping headsail or slot effect is necessary to obtain the "go fast" mode that you speek of. What has to happen however, is that air has to be moving over the sails quite fast. The backstay tensioner was hardened and sails were trimmed flat and sheets were pretty tight. What seems obvious to me...is that to get a sailboat in the go fast mode... drag has to be minimized. A worn out, loosely trimmed or loosely sheeted sail means drag and drag is tough to overcome. Of course, in lighter air, fullness is important to get a thicker foil and power from sails with minimum air flow offered.

I think it takes a significant boost in power to overcome hull speed but the sail plan on the c25 and c250 is capable of it given the right conditions. Another clue to the need for significant power to overcome hull speed is experienced downwind or on a broad reach. With a strong quartering wind... when surf is large enough... the boat will surf down and very often exceed hull speed, the extra down hill effect providing the boost.

Spinnakers boost a hull over the threshold...but once again, there is no drag, I mean airfoil drag... the chute is drag but putting it to work.

Arlyn C-250 W/B #224
<img src="http://www.stewartfam.net/arlyn/rr.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.cox-internet.com/arlynstewart/"]Arlyn's C250 Mods n Cruisin Stories[/url]

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Dave B
Admiral

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Mali
863 Posts

Response Posted - 06/17/2003 : 23:41:23

I suspect that a major reason an outboard with ample power struggles to reach the speeds that the sails can reach (or never does) is that as you apply power with the outboard, the force causes the stern to "squat", generating a stern wake that is substantially greater than you see under sail. Wakes require power to generate, and therefore eat up power from the engine. The more rolling swells you see behind the transom, the less efficiently your engine is pushing you through the water.

Dave Bristle, 1985 C-25 #5032 "Passage" SR/FK/Dinette/Honda in SW CT

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 06/18/2003 : 15:11:49

I suspect that, in a fresh breeze the sails generate far more than 10 horsepower AND the boat handling benefits greatly from the center of effort moving forward (from off the stern to the mast). As the boat heels and hardens up, the keel and rudder are generating lift - which doesn't happen under power. You certainly don't feel that characteristic tiller vibration when powering - even near hull speed, compared to making 4.5 knots upwind under sail.

Since you can clearly sail faster than you can motor, it takes a powerfull winch to trim the genoa and a mainsheet with 4 to 1 (or is it 8 to 1?) purchase to trim the main there can clearly be a lot of force on the sails - perhaps as much as several hundred pounds of thrust.

I don't know how this translates into HP, though.

<img src="http://www.indiscipline.org/cat25/pictures/icon.jpg" border=0>Indiscipline 1978 FK #398

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