Boat bottom and effect on speed

[beene]

I have been hurting for top end speed with my config all season. Thought it was a problem with the motor, prop, trust hub, gearing, engine slipping.... I have tried just about everything getting to the bottom of this problem.

Engine runs like a top.
Speed is identical to last season at 1500rpm = 6-7 mph
As soon as I throttle up, rpms climb far beyond what I was able to attain last season, and top speed is way down.
So.... engine is screaming, not going very fast, rpms are off the charts, rev limiter is kicking in, water trail from prop area very white and not clean....

I am thinking it is due to me.... el stupido.... I am trying out a new anti fouling wax this season. I applied it quite liberally and in my infinite wisdom, decided that I would not buff it up because I wanted as much protection as possible. In my convoluted point of view, I thought that buffing would only reduce the amount of wax on the hull, and I wanted as much as possible, so no buffing. Sure I new that not being as smooth I would loose some speed.... but not like this...

What I think is happening is the hull is so gummed up with scum, thanks to my stellar waxing job, that it is creating a mess of disturbed water at higher speeds so by the time the water reaches the prop, it can't do its job. The prop is therefore just screaming at high rpms with no smooth water flow to bite into.

I am going to thoroughly scrub the bottom and report back.

Anyone have any experience with a significant speed loss due bottom fouling?

Cheers

G

[Richard_Norman]

I think your answer lies in this sentence:

So.... engine is screaming, not going very fast, rpms are off the charts, rev limiter is kicking in, water trail from prop area very white and not clean....

If your rpms are highter than you COULD get before, you probably have some slippage in your power train. Thrust washer or something like that. If the bottom was causing drag, youe engine would be straining and the roms would be lower.

[Hamin' X]

I think Beene's point about the rough hull war that it was disturbing the water and allowing the prop to cavitate. IMHO, unlikely. However, he could buff it out, just for the hull of it.

~Rich

[craiglaforce]

You might have spun the prop hub.

New prop might fix your problem.

[dennisneal]

beene,

I'd hate to think that a simple wax job could really affect the boat speed of a Mac by maybe +/- 1 MPH. It might be important in the America's Cup, but, otherwise, I think it would get lost in the "clutter" of other conditions.

I agree that you should look for other causes for your speed problem.

[Capt Capsize]

I agree the bottom condition is probably not the major contributor. I looked like the previous owner of my 97X cleaned the bottom with a chainsaw. It had large area abrasions and deep gouges. After epoxy glass patching an fairing work the bottom is now much smother. The change added about 1 to 2 mph to my top end speed with the same RPM and prop.
Good Luck

[Phillip]

I wonder if you shouldn't visit iboats.com and go to the appropriate forum.
There are people there with the knowledge you are seeking.
Cheers
Phillip

[beene]

All good points, thanks to everyone.

I have changed the prop, the hub kit, checked the engine over, all nil change. My top speed was down 8mph at the start of the season form last season, and now another 5 mph less.

I bought my motor, prop, hub kit and just about everything that goes with it from iboats.com

I will go onto their forum and see what ideas turn up.

Will report back when I get this issue sorted out.

Cheers

G

[Hardcrab]

Strange indeed.
Is the engine tilt really and truely all the way down?
Have you changed the weight distrubition towards the bow, and the prop is now closer to the surface?
Does it matter if you are ballasted or not?

Perhaps the new prop hub is faulty also? Unlikely, but still ---
The ugliest thing possible could be stripped bevel gears in the lower unit.
Any metal chips in the lower unit oil?

Strange indeed.