help with new boat setup

[zul8tr]

Made two runs to the river today, First run I had the new Tohatsu tach hooked up and now realize this I'm checking the rpms with the motor raised just one bolt hole and my rpm's were about 5300. Second run I then went back home raised the motor to the third hole and was able to then get a solid 5400 rpm's maybe a spec more, Cooling was fine I kept looking back to make sure and it was good. Did notice two little cool rooster tails shooting upwards off the cavitation plate.

I really didn't notice any drop in speed going up to the third hole although didn't have my gps with me. I did notice looking back though there wasn't as much as a wake like before when I was only up on the second hole. Didn't hear any sounds like the prop was cavitating or anything.

I think I can go up to maybe the last hole but will really have to watch and see if it ends up sucking air and loosing water pressure, thought about putting a piece of tape at the top to holes even though there not open just to make sure it doesn't suck any kinda air.

I realize now a Jack plate is probably better at this stage I could probably run a little higher then on the last bolt hold and still be having good cooling and hopefully more speed

What you guys think? try a jack plate now? or go up to the last hole?

The motor needs to run at 5850 is what the manufacture says I was told it may even run a little higher that around 6100 is probably when the rev limiter will kick in.

prop used is 11x14.1 almost like i need to go down in the number which don't want to I want to go faster.

If your tach is accurate first go up higher w/o jack plate (less costly approach) and note the rpm results bring the GPS so speed is measured , w/o that results are not complete.

If still below the rpm target you need to go to less pitch. Less pitch does not necessarily mean less speed because the rpm is increasing.

based in your numbers if rpm is 5400 then your est speed with no slip
= 5400 x 14/(1.85 x1056) = 38.7 mph
if you had GPS speed slip can be calculated

Example if pitch is reduced to 13 and rpm can get to 6100 with jacking and trim (just an example) then;

estimated no slip speed = 6100 x 13/(1.85 x 1056) = 40.6 mph, up a bit

Note these are made up numbers but you get the idea.

Test and take notes on gps speed and rpm so we have a solid point of reference to calc slip and other.

There might be some benefit to reduce the diameter and keep the pitch but if the hull is heavy this will go against you to plane especially if water skiing is to be done. For pure straight line speed runs the smallest diameter and largest pitch that will plane the boat, reach the rpm and max speed target is what you want. However the boat may not turn worth a crap, so you have to decide what you want. There is only so much you can extract from the boat and motor for the hp and weight that you have.

[chris3298]

thanks zul8tr I'll probably raise the motor again and go tomorrow and see what happens and will bring the gps.


UPDATE,



I went back to the river again, this time the motor lifted to the 4th hole and the rpms was really just a spec over 5400 rpm's it wasn't much difference it seemed... cool little rooster tail went higher making you think you're going faster but not really I think the motor could go maybe another inch or inch and a half before i end up with a serious cooling problem. I was nervous this was to high but after putting a straight edge it really wasn't high enough.

My speed was 35.8 mph with the motor all the way up. Tomorrow gonna go do some fishing and will lower the motor back down a hole and see what my mph is from there.

I did notice some times the prop was slipping more when I was running WOT now i would think if it was slipping I'd get more rpm's correct? but they pretty much stay just a hair over 5400 rpm's

Started wondering how accurate this tach is but this is the tach from Tohatsu and it's brand new and is for this motor. I wanted to go with this tach rather then the tiny tach hoping it would be more accurate, no doubt though going higher up on the motor it does just sound better for sure and I would think the tach must be about right because it does have a rev limiter and have not heard it kick in yet on the motor so maybe the tach is fairly accurate.

Now what? go with a jack plate and maybe get the rpm's a little higher and then go see Hopkins about a propeller or what? I know of people who just go see him right away with out even testing to see what rpm's there motor is running but I would think it is important to do this so you're helping him and yourself of getting everything you can out of that prop. There not cheap.

Thanks for all the help everyone, I decided to kinda keep this thread going in hopes it may help others who are joining the forum.

Forgot to add I want straight line speed is what I'm after, my knee boarding days are over.

Chris

[chris3298]

Went with a power tech 10.5x13 NREB prop it turns 5500 rpm's and with a lot of porpoise I was able to get close to 5550 at 37 mph max speed.

I was trying for 5850 rpm's, aluminum 11.1x14 prop was a max of 5400 rpms at 36.2 mph

Thinking of trying maybe a 11 pitch prop only because someone on another forum has EXACTLY the same boat and motor setup Same brand boat and motor and all and he has this prop but reworked to a 12 pitch and cup added so it wouldn't porpoise so much and turned 5600-5700 rpm's This prop does porpoise more for sure.

Also notice the dang motor wanting to turn the boat more to the right but maybe this is because I had 100 rpm's more and it as the torque doing it.

Gonna go see Hopkins next week sometime and see what else we can try

[chris3298]

ended up bring my 10.5x13 prop back to Hopkins and I decided to keep this prop but let him cut the blades for my rpm's which I was able to get exactly 5850 or even a spec more. Forgot the GPS again so not sure of speed, it did feel faster though. Trying to work this dam porpoise thing out but i think if i can get this worked out so that I don't have to bend those trim tabs down so much I would think I should be at or dam close to 40 mph. motor is where I want it, if I can only get the boat to work with.

I'm debating on cutting the old tabs off and putting wider ones on or do some kinda adjustable trim tab not sure yet.