Why is my boat so slow under power?

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Golden Handcuffs
Posts: 41
Joined: Tue Aug 12, 2008 7:46 pm

Why is my boat so slow under power?

Post by Golden Handcuffs »

My GPS indicates that my boat goes about 4.2 knots at 2100 rpm, and barely gets 5.0 knots at 2600 rpm under favorable conditions. From what I read, I should be getting at least 1.0 to 1.5 knots faster than this. Any suggestions on what I should try? My boat has the new "M" series prop, a clean bottom, and isn't particularly loaded down with anything. The only think I can think of is that my tachometer is out of calibration (can that even happen?).
Tim April IV
Posts: 78
Joined: Wed Nov 02, 2005 12:24 pm
Location: Seattle

Re: Why is my boat so slow under power?

Post by Tim April IV »

Clean bottom and appropriate prop should push a C350 well over 6knots in reasonably calm conditions.

You ask about tachometer calibration - folks will tell you that they are notoriously out of whack.

You should be able to determine what the tach reads at "WOT" - without the engine in gear. "WOT" is Wide Open Throttle. Simple make sure that the center button on the throttle/shift lever is pressed in and run the engine RPM's up to WOT - and see what your tach says. A few minutes at WOT isn't going to harm your engine. Then, underway, see if it will reach the same RPM's while in gear and pushing the boat. Again, a few minutes of WOT isn't going to hurt anything. If you aren't able to achieve near the same RPM's in gear as when you check it while out of gear, suspect the prop.

Normal cruising RPM's should be about 85% of whatever your WOT number comes out to be.

Observe the exhaust color (white, black, etc) when doing your short WOT test while in gear. If you see significant color coming from your exhaust, make a note of that as well, then check in with a diesel expert. They can tell a lot from the data you collect doing the above things.

Good Luck!

Tim Brogan
April IV C350 #68
Seattle
Tim Brogan
April IV C350 #68
Seattle
Golden Handcuffs
Posts: 41
Joined: Tue Aug 12, 2008 7:46 pm

Re: Why is my boat so slow under power?

Post by Golden Handcuffs »

I have not directly checked the calibration of the tach, but I really don't think that's what the issue is. When I open up the throttle all the way, the engine tops out at 3000 rpm, right where it's supposed to. Someone suggested perhaps t of an air or exhaust problem that might be limiting my power---that's an interesting idea. However, I would imagine that if I wasn't getting full power (all 35 horses), the prop would be overloading the engine, and I would be seeing black smoke form the exhaust. I never see any smoke from the exhaust---black, white or otherwise. I will consult the Nigel Caldor diesel book and see if he has any ideas. Based on my limited knowledge, the only conclusion I can reach is that the boat is "under propped"; that the prop is not big enough or pitched enough to take full advantage of the engines power. That would seem really odd though for a boat with a propeller that was tailor-made for it.

Who should I call on this? Catalina, Michigan Wheel, a boatyard, a prop shop, a diesel shop?
RCattell
Posts: 18
Joined: Wed Jan 09, 2008 8:09 pm
Location: St. Petersburg, FL

Re: Why is my boat so slow under power?

Post by RCattell »

Golden Handcuffs,

We have hull #451, and at the last haul out the prop went off to the prop shop.

What they found was a large imbalance of 12 grams, and pitch was off from spec.
I had the prop shop dynamically balance the prop and set pitch at 10.75, up from 10.
They got the balance down to less than a gram.


The boat runs smoother and a bit faster, and the cost was around $180.00.
My friends 26 year old Sabre had his prop done as well and his was only 6 grams out of balance.
These props are made in China is what I was told.

I also agree with others, you must check your tach and calibrate it, they are notoriously off.

Hope this helps,

Richard
Richard Cattell
S/V Rachael Lynn
Catalina 350 Mk II
#451
Golden Handcuffs
Posts: 41
Joined: Tue Aug 12, 2008 7:46 pm

Re: Why is my boat so slow under power?

Post by Golden Handcuffs »

Well, I finally got my tach checked, and apparently it was way, way off. I haven't got the final word from the mechanic, but I think they may have adjusted my throttle cable as well. When I was out this weekend, the boat went much, much faster! I was no longer getting passed by small daysailers with outboards! My dignity was restored.

I'll report back, but I think they must have done more than just adjust the tach, because of what the engine sounded like--to my ears, it sounded like it was running faster at 2500 rpm than it ever had at wide-open before.
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