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 Boat speed		  New Topic  Topic Locked
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Don White
Deckhand

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3 Posts
Initially Posted - 09/09/2008 :  11:19:55  Show Profile
Question. What speed should I expect on my C-25 fin pushed by a Johnson 9.9 Sailmaster? Compression is good, new plugs, and bottom is in good shape. Don
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Prospector
Master Marine Consultant

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Canada
3159 Posts
Response Posted - 09/09/2008 :  11:22:24  Show Profile  Visit Prospector's Homepage
On ours we can get up to 6.2 kts.

Comfortable without having th emotor sound like it is about to explode is 5.5 kts. I think our engine is the original Johnson 9.9 from 1984.
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 09/09/2008 :  12:08:17  Show Profile
Above about 5.5 knots, the boat begins to squat so the transom is creating more wake, and therefore drag. Above 6 knots, you're beginning to try to climb over your bow wave (what essentially defines the "theoretical hull speed" of 6.3) and go into semi-displacement mode (bow up like a trawler). The result of either is more fuel and noise per mile. I kept my Honda 8 between 5 and 5.5 knots through the water--it seemed like somewhere between half and 2/3 throttle--which currents sometimes translated to 8+ over the bottom. At that speed, I burned a little over a half gallon per hour... I suspect the extra 3/4 knot would cost me 30-40% more.
Edited by - Dave Bristle on 09/09/2008 12:13:54
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Voyager
Master Marine Consultant

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USA
5418 Posts
Response Posted - 09/09/2008 :  22:17:30  Show Profile
Dave - you're correct. I've done the 30 or so miles four times between Stratford and Westbrook now, and when I keep her around 5kts (knotmeter not VMG), I get very good mileage. When I try to crank out the extra knot, I can count on about 1 gal per hour.
The difference in noise level is quite marked. I can hear my FM radio at 5kts, but no chance at 6.
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jerlim
Master Marine Consultant

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USA
1484 Posts
Response Posted - 09/10/2008 :  11:26:23  Show Profile
Same results w/ our Merc 8hp. Dave - glad to learn you had an 8, I've been concerned that we've been underpowered.
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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts
Response Posted - 09/10/2008 :  17:37:23  Show Profile
Can't speak to the Merc 8, but it's been said that a high-thrust 4-stoke 8 is roughly equivalent to a 2-stroke 15 for pushing a heavy boat... It probably is in terms of weight!
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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts
Response Posted - 09/11/2008 :  13:28:50  Show Profile
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by jerlim</i>
<br />Same results w/ our Merc 8hp. Dave - glad to learn you had an 8, I've been concerned that we've been underpowered.
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

For the past 7 years, I've been using an 8hp Merc on my C25 in just about every imaginable weather/sea condition, soft groundings, and river currents, and I've never felt I was underpowered.
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Dave5041
Former Mainsheet Editor

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USA
3758 Posts
Response Posted - 09/14/2008 :  00:29:02  Show Profile
9.9 is a waste of horsepower on our boats. The only reason I have one is that it is the lowest horsepower I could find in an XLS with electric start. When your stern squats you are just wasting gas. 8 hp is 8 hp, power curve and gear ratios in combination with prop pitch and blade number and diameter determine thrust. My Mercury Bigfoot is geared down more than the standard Mercury 9.9 and drives a large 4 blade prop at lower rpms, making it perfect for slow, heavy boats. I am sure that a proper 8 and probably a 6 with appropriate gearing would be perfectly satisfactory, if it were available. I have been on an Oday 23 pushed by a 4.5 in very unpleasant conditions, and it was definitely adequate.
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bigelowp
Master Marine Consultant

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USA
1797 Posts
Response Posted - 09/14/2008 :  08:52:55  Show Profile
I would tend to agree that a 9.9 4-stroke is more power than needed. In a variety of conditions on long Island Sound I have never had to go over 1/4 throttle! Can't speak regarding a 2 vs 4 stroke other than I have also heard that the 4 strokes generate more torque and are heavier. If I could have purchased a 25 inch 8 HP Tohatsu I would have, however that said I do enjoy the relative quiet of using my Tohatsu 9.9 at lower rpm's . . .
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ClamBeach
Master Marine Consultant

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3072 Posts
Response Posted - 09/14/2008 :  18:44:19  Show Profile
"4 blade prop at lower rpms, making it perfect for slow, heavy boats. I am sure that a proper 8 and probably a 6 with appropriate gearing "

True enough. It's the gearing and propeller that let the engine put the power to the water.
Here's a Youtube video of a 5hp that I'm sure would drive a C25 quite nicely.

http://www.youtube.com/watch?v=q7l_Vy9BfG4&feature=related
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josgood
1st Mate

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USA
57 Posts
Response Posted - 09/15/2008 :  09:56:30  Show Profile
I have the (new) Honda 9.9 and even at 9114 ft above sea level it is more than enough. I would not ever look back though... I absolutely love it !!
keeps the batteries fully charged, extra long shaft... and it always starts with one little push of the button.
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.