Motor mounting Height - Huge Difference -

[gran398]

Agree, think you'll need to raise her up to achieve 5500 rpm min. WOT. Check again the pitch. The stainless raker-style props will add 4 mph. on the top end, based on feedback.

[wingtime]

I have a 17" pitch prop I want to try but I'm going to raise the motor first and run it with the same prop to see what the results are before I change the prop.

[GoneFission]

I think a 19" stainless would be about right - get the motor in the right hole and you should get over 5000 RPM trimmed out at WOT.

[fitz73222]

Please dont miss understand this statement... I see some dart throwing at setting up this boat. Here are some basics. All outboards are dyno tested at 77 degrees and 50 % relative humidity to establish horsepower and create the standard per NMMA test method. That means, that do to atmosphereic conditions, engines will gain or loose horsepower based on the temp and humidity. You are testing this engine set up in the middle of summer with 90% humidity. Typically, the difference between winter and summer is 250-300 rpm. I have seen this with every boat I`ve owned and year after year this holds true. This can mean the difference between being over propped and under propped on a boat with moderate horsepower such as yours. If your engine has an WOT range of 5500-6000 as an example; the standard rule is to prop her at 50% or higher in that range. So you want to see 5750 with a NORMAL load and slight positive trim. That means, the wife and kids, 50% fuel, coolers, top up, whatever... Not, you, 10 gallons of gas, top down, and no extras. Propping a boat correctly, is about saving the powerhead, not seeing what she will do in the quarter mile! Since you are setting this up in dead of summer, prop the boat to obtain atleast 5600-5700 today. In the winter, the same load will yield 5800-6000. If your playing with propellers, stick with one brand and pitch up or down accordingly. All props are designed differently and bite differently. I like Stilleto propellers and they historically run about 8.5-9% slip, which is excellent bite. Dont get hung up on propeller rake. (the blade angle from the root to the blade tip). Rake is designed to increase bow lift on lighter, faster boats. I could go on and on but I have to get to work!!

[Aswaff400]

seems like what type/size of prop one should use on their boat tosses up about the same amount of arguments as what kind of oil/spark plugs i should use in my truck  

thats why its called test and tune, find what YOUR boat runs best with and run with it...

[wingtime]

Fitz good point on the temp effecting power output during summer.  I'm fully aware of that fact and in aviation we call that density altitude.  I hadn't really thought about it too much for the boat though so I'll take that into consideration.  I don't see any dart throwing going on.  I had the boat loaded with myself, half fuel and two heavy passengers (over 225 lbs each).  The motor is clearly mounted two low for my boat.  I can barely see the "tail" of the midsection above the water while on plane and the cavitation plate is fully underwater.  I'm going to raise the motor up and go for another test/tune.  I could care less about the speed of the boat.  It's way faster then it ever was now and this hull is not going to go fast anyways.  My main concern is getting my WOT RPMS right for as you say to protect the power head.  My motor is rated for 5000-5600 WOT and my goal is 5300.

[fitz73222]

Hey Wingtime,

Mercury does an excellent job of explaining it in just about every publication they have pertaining to service and set up. Very informative on air density, barametric pressure, temperature and its effects on horspower and propping. I`ll be glad to send you a copy of the publication if you send me your email address. I`ll scan it and send it to you.

[GoneFission]

I'm just not a big fan of running the 2.4 liter Mercs up to about 6000 RPM.  I know some folks say they will run all day at 5800-6000, but I've also seen a lot of 2.4 blocks with big holes in the sides.  

I would stay with a prop that runs 5500 tops and have a little margin of safety.  If you want to run 6000 in a 2.4, consider upgrading to the newer model connecting rods.  You could also go with a 2.5 that by reputation has stronger rods.  Just my $0.02.  

[fitz73222]

I think his is a 2.5, but anyway I`ve run 2.4 `s on  Allisons at 6800 rpm`s with stock rods. Most of the issues we saw were broken rings on the bridgeport version blocks and an occasional burned piston on the early Bosch fuel injected designs. The only modifications we ever did to the blocks were take them apart and clean up the ports and intake channels to get them to breathe.

[wingtime]

Yeah mine is the 2.5.  I love how it sounds...

[slvrlng]

Hey WT kind of like this?

http://www.youtube.com/watch?v=wy_lpli2 ... re=related

[wingtime]

Hey WT kind of like this?

http://www.youtube.com/watch?v=wy_lpli2 ... re=related

yeah I've seen that video.  Crazy dyno set up that thing is on.

[Skoot]

Well, I havent been "super" happy with the height/performance of my motor so I finally raised it to the last/final hole today. (Im gettin good at this)  It was defiantly better when I raised her 2 holes, but not perfect yet.  Gonna test her out tomorrow.

Wish me luck.

[slvrlng]

You sir have been working entirely too much!

Good luck and hang on tight!

Get your better half to shoot a video of that Tohatsu at full song!

[slvrlng]

And.......... We're waiting!!!