Loss of speed in C36

[Starboat]

Problem is in a Catalina C36 with a Universal M25. Boat has a maxiprop 15x11; when first installed, boat would easily motor at 6+ knots (push to 7) but last fall couldn't get boat much over 5 knots. Had boat pulled, new paint, repitch prop; worked fine. After sitting over winter, experiencing same loss of power problem...have to take engine to almost 2600 rpm to get near 6 knots. Any ideas/suggestions?

[senormechanico]

It sounds like a dirty bottom/prop (again) to me.

Let us all know when you discover how to keep everything clean for 6 months at a time. We'll all be glad and you'll be RICH!

Steve B.

[Alan Wheeler]

Growth on the bottom sounds like the issue. It doesn't take much to make major friction through the water. Even just slime will greatly reduce your speed.

[senormechanico]

Just to put a fine point on how much difference a smooth bottom can make, I documented the difference a couple of years ago on my 33' trimaran. I was going to do a super smooth bottom job, so first I pressure washed the old bottom paint. Then I launched the boat, motored up and down our canal system at exactly 2500 rpm during slack water. I noted the speed during the passes and averaged the number.

After doing a month long super smooth bottom job which included burnishing the paint and using a slippery coating, I launched again and repeated the slack water speed trial at the exact same engine rpm.

The boat went a full 15% faster with the new bottom. No other differences.


Steve B.

[GordMay]

Steve's results very closely match the experience of the US, Cdn, & Au navies;
“Prevention of Marine Growth on Naval Vessels”
Goto: Prevention of Marine Growth on Naval Vessels
“All naval vessels require clean, fouling-free hulls to maintain operational effectiveness and to reduce fuel consumption caused by increased drag due to an accumulation of fouling. An effective antifouling paint can produce fuel savings of at least 15% due to reduced drag compared to an untreated hull …”

[Alan Wheeler]

Which goes to show that it is better to spend the little extra on a real good anti-foul paint. It will well and truely pay for itself in fuel saving.

[Starboat]

Thanks for replies.... I plan to take the boat into fresh water lake for July 4 weekend that should help clean bottom; however, I have looked at the bottom and it looks quite good (confirmed by diver). It will be interesting to see if a few days in fresh water improves things....

[Alan Wheeler]

Depends what the growth is. The fresh water will kill many marine animals and plants, but it won't remove it if it is well adhered to the hull. Plus hulls tend to be slower in fresh water than Salt. So you will alos notice a slight reduction of speed in the fresh water lake. Altitude also affects the speed.