Here's a head scratcher...

[Jan from Humboldt]

Last year on flat water going WOT (About 6000 rpm) I was able to get 29 mph on the gps and +30 on the boat spedometer.

This year, same boat, same load, same rpm and per the gps its about 25-27 mph.

Yes, I checked to see if the anchor was out , and the prop is the same as last year and no damage to it that I can see and it was re torqued two weeks ago per the specs (@ 29.5 ft lb).

This isn't a serious issue as I don't run WOT a lot, and the boat goes real nice at 4800-5200 rpm, more so a curiosity item as to the loss of a few mph of top end speed for no outward reason that I can see.

any ideas out there???

[suitekids]

Maybe the rubbber hub is slipping between the prop and the splined hub, torqing would have no effect on this. have the prop checked, or borrow a similar one to re-test at WOT

[Pip]

A rather elementary answer but if the boat is left in the water "crud" can build up on the bottom of the boat which can slow the boat speed considerably. I do not know your situation but just thought I would throw that one out there.

Happy and safe boating!

[Jan from Humboldt]

Thanks, the boat is on a trailer so bottom gunk is not the issue, but slippage might be.

[ksr5]

It might be ethanol less gas mileage less horse power just a thought.

[Jan from Humboldt]

Originally Posted by ksr5

It might be ethanol less gas mileage less horse power just a thought.

I don't think so as I was using E10 fuel last year, I'm thinking that I may possibly have a prop problem, either with some slippage at high rpm's or the prop is flexing or has flexed and thereby changed it's pitch.

The slippage I can check by a simple marking of the prop with a sharpie indelible pen and see if the marks change.

[MolarBoater]

Add octane booster and try it again or a new fresh tank of gas. I would think variation in "freshness" of gas either in the boat or out of the gas stations' tank or worse...marinas'. Other additives could improve performance (fuel injection cleaner) or hinder slightly by their nature.

When was the last time the fuel filters were changed? Motor oil? Or the gear oil? Or the spark plugs? Timing checked? Could be a deteriorating fuel line or bulb.....soooo many things......let us know what you find out.

Was it windy? choppy? How was the current running? Were you standing up or sitting down? haha, Molar

[Jan from Humboldt]

I did a "Mid season" P.M. routiene on the boat 3 weeks ago all filters and upper and lower oil changed, the shop also did a complete carb rebuild in early May (Gotta love the E10 fuel) so plugs, timing are all good. The tank of fuel was very fresh as I buy fuel at a local station that has a high volume.

The motor is a 2005 and has 75-100 hours on it, the speed testing was done in both directions on the bay. Wind was 8 knots out of the north and I was sitting, motor trim was properly adjusted for optimum speed.

Yeah I know about those variables but nothing much has changed that I can see that would drop the top end like this, I suspect that it might be the prop slipping but I can't test that till I get back out on the water.

[MolarBoater]

Hmmmm, why would the prop be slipping? Did you run into some shallow water or something? Would that cause "slipping"? I have run thru oyster beds and plenty of 2 foot areas, haven't noticed prop slipping but maybe not looking hard enough for it.
good Luck and let us know what you find out.

[mhedrick]

Just a thought, but could have water in the hull. Extra pounds= less speed.

Also, your gps is not reading KMPH instead of SMPH is it?