'86 115 I6 needs more speed

[redjmp]

I have a freshly rebuilt 86 115 on a 15' Glascon speed boat (cobra/cvx style)
I think the hull weighs in at around 900lbs as it tips the scales with motor and trailer etc at 1600lbs. With me and fuel etc in the water it probably also weighs 1600 1bs.
Cannot find pitch on prop but it is aluminum 3 blade slightly cupped but not as much as most ss props.. Gps 42 mph at 5800.
3500 @ 27mph gets best fuel economy of 2.5 km/liter (5.87 mpg) and I tend to run it there for cruising.
It has an amazing holeshot that I would live without if I can get more fuel economy and/or better top speed.
Any suggestions would be welcome.

 

[WillyBWright]

Re: '86 115 I6 needs more speed

Without knowing where you are, it's hard to know where to go. Whatever the pitch is on that aluminum would seem to be the pitch to look for in a stainless.

 

[Texasmark]

Re: '86 115 I6 needs more speed

No doubt about it, you cannot get there with alum as far as I am concerned.

You need a HP SS if you want to move up with all other "setup" things equal. I think it's due to the blade thickness. SS can have thinner blades (and all the other stuff...porting, high rake, camber, cupping) and the thinner blades run with less resistance to the water. Like a baseball bat and machete slicing thru the water. BTDT.

So, as the big boys say, "get your setup right first", then get rid of the alum.

My 2C

Mark

 

[Bob_VT]

Re: '86 115 I6 needs more speed

I went through the same routine last year.

It really is all about set up. Play with the engine height and keep a close eye on your water pump..... I would highly suggest you add a water pressure guage to your dash.

 

[redjmp]

Re: '86 115 I6 needs more speed

Thanks guys. I took my prop off and sharpened it and sanded the paint off and it now does 43 mph. I had already raised up 2 holes because it seemed to kick up a rooster tail but found that the prop ventilated when cornering.
Lowered it down a hole and all is fine. PT&T makes short work of getting it trimmed.
The prop store here at iBoats says that WOT for my year 115 is between 4750 and 5250 rpm as opposed to 5000-5600 for other years of I 6's.
If this is the case then I'm way over that at 5800. Even if it weren't, I'm still over the limit.
Does anybody have the actual WOT range for my 86 mer 115 I6?(low dome) I thought it was built like the old 1400's minus the high domes.
Using the prop calculator, I'm comming up with about a 17" prop.
I'm thinking that I should try a 19" steel. Or maybe even a 20 or 21. I'm not sure how cupping affects anything other than adding 1" of pitch.
I'm not sure about rake and camber. How do these effect performance? Remember that this thing has lots of power to spare on this hull and I used to be able to pull a skier with just an old 2 hole lark 50.
What about diameter? 13 or 13 1/4 or 13 1/2 or even 14? How does this have an effect? What about 4 or 5 bladed props?
I see guys on here getting more than 50 mph with 70 hp 'rudes and 65 with a 140.
Shouldn't I be able to get something like 55 or 60 with my motor?
Thanks again !