running engine high on transm?

rednaell · 04-07-2008, 08:55 AM

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

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 hight I am running? What pitch and size of prop do you reccomend

Maxim Gibson · 04-07-2008, 01:08 PM

Will you be leaving the boat in the water then as opposed to launching and hauling out each time?
What kind of antifoul did you use?
a s/s prop isn't necessary on your setup, all I can imagine it doing is giving the engine more load and reducing the revs.
Do you know what rpm your engine should run at?
I imagine you could go up a couple of inches on the pitch but go to far and it'll just be under-revving.
What I'd do is find the optimum rpm for the motor to run at (at full throttle) and if it is running above this with the prop you've got go up in sizes until you find one where it will run at this speed but not allow you to over-rev.

If you still wanted to go for more speed (as we always do!) then you could look at lifting the engine on the transom and running steeper, larger props as your current engine height (looking at the cav plate) is similar to that of a typical pleasure boat, not something set-up for ultimate speed. However as it is only a standard lower unit you would have to look at low water pick-ups otherwise you would risk over-heating.

rednaell · 04-07-2008, 02:06 PM

Yep that`s why I painted anti-fauling. I am leaving the boat in the water this summer. I`am using Jotun hard type antifouling siutable for high speeds. However it`s quite a thick paint, and it does not give a glossy finish. That may increase the drag but surely not by that much.
Could it be that the prop is slipping from the hub ?

TheOrs · 04-07-2008, 06:37 PM

40mph with that boat/engine/prop combo seems very good. My boat (14ft Fletcher/75 Merc/13x18" stainless prop runs 42mph (speedo reading) @ 5500rpm)

Are you saying that the only difference between the two runs was the anti-fouling? Doesn't seem right.

Generally, on a pleasure boat, the anti-vent plate on the outboard should be level with the bottom of the boat. Yours looks higher than that. Higher gives more speed but less grip on acceleration. If you are running your engine with the ant-vent plate a few inches above the bottom of the hull (difficult to see for sure from your pictures) then a semi-piercing stainless prop like a Laser II might be beneficial. Otherwise you may have to cut the transom & drop the engine a little.

marshan162 · 04-07-2008, 09:02 PM

I can't see this being the paint, are you sure the prop is losing bite with the water and not the hub slipping.

Maxim Gibson · 04-07-2008, 10:08 PM

I agree, could well be the hub slipping.

rednaell · 04-07-2008, 10:40 PM

I am going to mark the hub with the prop and watch if it slips.
I will let you know

rednaell · 14-07-2008, 09:38 AM

The hub is not slipping! Mark still lined up. Therefore it is a ventilation problem.
I can not understand why the prop is venting so much. When it get passed 3000rpm, you can hear it grouling,(sucking air), and over-revs with no propulsion.
The prop is not that high, I cannot understand, why it is doing so. This motor is geared high, 1:1.64, and uses a small 10.4X15P prop. maybe the prop is too small in diameter for mounting close to the surface. It is not piercing the surface for sure; I checked it with a ruler aligned with the bottom. the prop is still under the ruler surface.
I am going to try a 4 blade Propulse composite prop (6902), maybe it would grab the water better.

what do you guys think ?

blind dog · 14-07-2008, 12:00 PM

hi,just wandered as your in malta whats happened to the racing out there as the web site has changed for months

TheOrs · 14-07-2008, 07:51 PM

The engine looks too high to me. The anti-ventilation plate should be level with the lowest part of the transom.

The prop does not have to pierce the surface for ventilation to occur. If the prop is too close to the surface it will suck air down in around it.

Have you got a close-up side-view of the boat with the boat level & the engine upright?