disappointing performance!

[leander borg]

Hi all. This is my boat 16.5ft, with a Force 70hp (4500-5500 wot). The prop is a standard aluminum 10x15p. Note the transom is high on this boat, I draw lines on one of the pics attached so to have an idea of how the hight and trim is set.

A week ago I made some boat trail-runs(with no anti-fauling). I was getting 40.6mph @ 5600rpm. Was quite happy with this

Yesterday I tried the same setup but now I had the bottom of the boat painted with anti-fouling.
I was expecting some drag and loosing a couple of mph obviously, but I was very disepointed with the new reaction of the boat.
The boat ran fine till reaching 4000rpm, then at this point the prop seems to loose grip of the water and the motor over revs to 6000rpm. Top speed 26mph.
Do you think it is neccesary to use a stainless-steel prop at the hieght I am running?
What do you think the problem may be ?

 

[marine4003]

Re: disappointing performance!

Your prop is losing traction..its called cavitation,its when too much air gets drawn into the prop,and as you said there's no traction,I'm not a prop expert,so i'd post this thread under props,im sure the folks there can steer you in the right direction..Great Boat.

 

[tommays]

Re: disappointing performance!

THE paint should not really matter unless you were in the water long enough to have slime start growing on the boat

 

[NSBCraig]

Re: disappointing performance!

Nice boat.

Your motor looks to high, your anti ventilation plate should be only about 1 inch or maybe two above the bottom of the boat.

You'll probably get a lot more from putting an jack plate on the back than buying a stainless prop. The set back will allow you to run the motor high like you have it (or a bit lower it looks real high) and still bite water. (of course I just gave one away)

Make sure your getting water through the motor! You can buy a transom mounted water pickup for $50 from bob's.

Anti fouling paint will definitely slow you down if you don't really need it it's a bad idea.

But if your not keeping it in the water all the time get it off that trailer. That thing is killing your boat. You need a trailer with nothing but bunks, not even keel rollers, bunks and keel bunks.

Not sure who made your boat but it should be a balsa cored light weight hull and you really don't want it sitting on the trailer.

40 mph with a force that's pretending to put out 70 hp is doing good. (I like the powered by mercury vinyl)

Was anything different besides the bottom paint? Weather, water, more people on board, more fuel?

 

[leander borg]

Re: disappointing performance!

No notting was different form the two runs same 2 persons and gear, same wind etc. Someone told me that it may be the propeller sliping from hub. So i`ll check for that first then I let you know.
I am expecting loosing about 3,4,5 mph maximum with the antifouling. what do you think?

 

[NSBCraig]

Re: disappointing performance!

Yeah your gonna lose around 5 mph with bottom paint.

Let us know if it is the prop spinning.

 

[reelfishin]

Re: disappointing performance!

The motor is way too high, either find a jack plate and lower the motor or find a longer motor.
What does the transom measure top to bottom? It looks like either a short shaft motor on a 20' transom or a 20" motor on a 25" transom.

I'd also toss that trailer for something with more support, sell it to a vintage boat guy or something, it looks like an old Holsclaw or similar.

I don't really have anything against Force motors, but they don't make near the power their stickered at. That motor is probably more along the lines of a Merc 50 or so.
What is the boat rated at?

It basically looks like a case of the wrong motor for that boat. You can make it work but you will spend less finding another motor and probably be happier in the long run.

 

[5150abf]

Re: disappointing performance!

Look at a manual jack plate, probably the cheapest fix, the motor is just too high, as the boat accelerates and comes on plane it pulls the motor out of the water and it sucks air, nothing to do with the prop or paint, it is just too high.

A jackplate will alow you to get it down and fine tune it, once you do that you may do even better than 40.