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I have looked at a lot of the threads here and not quite found my answer. I have lots of experience with engines (gas and diesel) and other auxiliary gear, but not so much with tiny little outboards. I also have never kept a boat in a slip for extended periods. So, here is my question:

I have a brand new (got in April) Mac 26M with a Honda BF50. I have ablative (read: not very smooth) bottom paint on the boat and keep it in a slip.

From day 1, I never got more than about 17 knots (19.5 mph) out of the boat, and it would drop to about 16 knots with ballast full. I figured I was not getting the advertised 20-22 mph due to the bottom paint slowing it down.

The "stock" prop that came with the motor is a 3-blade aluminum and is marked as 11.75x10.

Most of the time I motor just to get out of the basin, but I do usually open her up to w.o.t. for a minute or two after a period of running at 5-7 knots, just to "blow out" the carbon. Generally I don't pay attention to the speed though, or I haven't been...

The other day I actually had to motor for several miles due to the wind dying, and found that my max speed is now 12 knots (ballast full)! The motor is still developing 5500 rpm w.o.t. (red line at 6000) and the prop is pristine. I had the ballast full, the DB down about 6 inches (for tracking) and the rudders fully up. No substantial load in the boat and only myself (200 lbs.) on board. About 20 gallons of fuel in the tanks.

So, any ideas what is going on? If I have a clogged fuel filter or something, the engine would not still reach 5500 rpm would it? It's been in the water (salt) for 2 months and it doesn't "look" like there is much, if any, scum on the bottom, but could this little layer of scum be enough to cost 5 knots?

Thanks,
AndyS

I was amazed at the speeds you were first achieving w/ just a Honda 50 - especially one that's painted! My experience parallels yours ... as the boat ages, speed drops even though rpms remain up. Can only guess that the factors are:

* Hull is dirty
* Engine loses some power as it ages
* Boats become heavier over time (we all have this problem?)

My Suzuki DF60 was able to run at 23-24 mph wide open. During the first season, that slowed to 21-22. Now it's 20-21. No paint, boat is dry-stored. Aluminum prop (14x13) is aging too. C'est la vie?

Frank C -

I agree, things will slow over time to some point. But you lost like 3-4 mph over more than one season - I have lost 4-5 knots in 2 months! I do hope these little outboards don't age that quickly!

- AndyS

Andy, I would guess you have more growth on the bottom than you think. It recently happened to me and I was losing about 2 kn of speed. On the sides, just a little slime and a few small barnacles. But on the very bottom, all around the CB trunk, I had close to 3/4 of an inch of hard fouling. This mostly happened because my boat got beached in the sand by accident one day a couple months ago. Sand scrapes off the bottom paint in a hurry, then add 85+ degree Florida waters to it, and its a recipe for major fouling.

Btw, the fastest I've ever gotten my 2000 Mac with Merc 50 BF is 18.3 mph, that was with a clean bottom. Recently, I've taken off the hydrofoil fins that were on the engine so I'll test again once I finish scraping and re-painting the area around the CB.

Your dagger may not have been painted and if your leaving a ft in the water you have a ft of barnacles. Yes antifoulant paint will slow the boat down about 2-3 knts if it's just rolled on. And Newport is a healthy growth area as well. Take the boat over to T Warf on the south side of prudence. There is a beach area where you can anchor the boat in 4ft of water. It has southen exposure so do it before the seas kick up. Take a brush and clean under the hull.

If your getting 5500 rpms your motor is doing fine.

With the centerboard down even a little on my Mac26X the top speed was reduced by a 1 to 2 mph.

Just to follow up -

Bob and Dimitri had it right.

I did some snorkeling and sure enough my daggerboard is not painted - even though I lift it all the way in between sails, it has an inch or two that stays wet and the giant gob of growth on that bottom little strip was amazing.

Once I cleaned off the daggerboard my ballast-full speed jumped back to about 14 knots, ballast-empty has me back up to almost 16 knots. This weekend I'm going to put it on the trailer and pressure wash the bottom before going on a week-long trip next week.

Thanks for the clues, guys!!

- Andy26M