too much prop slip

[johnson89]

1989 90 hp johnson... motor has had a lot of running problems but everything seems good now. 14x19 prop. 3 blade... motor turns pass 5600 rpms with 2 people but only getting 35.5 mph on gps...the best I saw was 38mph an that was 5900 rpms,,,anything I can try too cut back on the prop slip... hole shot seems great an have great bite in the turns. the prop is in good shape not 100% perfect.... thanks for anys ideas too try..

 

[steelespike]

Re: too much prop slip

We need boat details to help figure things better.I think 19" pitch is pretty typical depending on boat details.I think you need to varify the tach is accurate.Perhaps borrow a shop tach.
Unless the boat is light and fast 35-38 mph is reasonable.

 

[QC]

Re: too much prop slip

Yeah boat details and tell us about trim and affect on speed, and also prop condition can be huge . . . I am using a 2:1 gear ratio and with the rest of the numbers you've got (have to be perfect) I am getting 29% slip, so I can understand your concern. But we need dead nuts accurate info. Garbage in, garbage out as they say

 

[johnson89]

Re: too much prop slip

its a 1989 hydro sport bass boat 15'9 with a tape measure.. Tilt and trim,I can increase speed (rpm's) about 150 rpm's when I trim motor up.The hull is in good shape on the bottom....I don't own the gps,I had a friend with me today an says it is dead on(with his car speed)...I lloked up boat weight it it was 775, it has 3 batteries(small size) had 9gal. of gas in tank, both of us total weight about 280lb.sThis boat gets on plane in about 4-5 sec.s .....

 

[QC]

Re: too much prop slip

Take the hydrofoil off and try again. I trust the GPS numbers, I always want to know that the tach is right . . .

 

[steelespike]

Re: too much prop slip

Not trying to nit pick but I find the 16 to be 810 I think total weight is about 1600,
boat, motor,people, gear etc.
I too would suggest remove the foil.It is a bandaid and a compromise at best.
Removing the foil may slow your hole shot but could add a couple of mph and it may give better results when you trim.There are better choices for trim problems.Trimtabs like smart tabs could be a good choice.If your batteries and gas are in the stern; if you could move some forward even if just midway.would hehp.
I would suggest you get your prop repaired check your tach and get a new base line.
If results indicate a prop change you can keep yours for a spare.

 

[johnson89]

Re: too much prop slip

its going too be a few weeks before I can recheck things.I am sure its the original tach that came with the boat , have never had any problems with it but not 100% it is dead accurate...I will try it without the hydro-foil on my next trip, the boat came with it when I bought it 9 years ago.This saturday is the first time i even checked the speed an most people I talked to says I should see at least 40.I don't really care about being able too do 40 but would like too have things right so my MPG will be better.I will also check and make sure the motor is up high enough, right now it is dead even with the bottom of the hull.I am not sure how many rpm's the motor will turn this prop,I had too adjust the stop screw too keep it from going any higher, right now it will go to about 5900 so It might go as high as 6100 or 6200, I don't run it pass 5600.......

 

[johnson89]

Re: too much prop slip

well laid off from work this week so spent some time going thru the boat an decided too pull the tach out. it had a setting dial marked in this order 2,3,4,6 and 5..it was set at 4.I found out under the johnson section that this should be set too 6.I took the boat out today (no gps) an opened her up.I got 5400 rpm's and the speed I am not sure but was the fastest I have had this boat at.And it was a windy day an water choppy so it might still do better than today run.My idle rpm's seem a little better also got 700-750 on gauge instead of 850.I did raise the motor another 1/2 " an all is good, might be able to get another 1/2" not sure.After finding the tach. problem I left the fin on for now.Got too check everything with a GPS too see how much the prop slip went down,I am sure it went way down...Thanks for some of the ideas on what to check...

 

[steelespike]

Re: too much prop slip

Nice find on your part.
When I see crazy slip/performance based on numbers I allways think it's only as accurate as the input especially regarding slip.
Raising the motor probably reduced the drag from the foil as well as the motor.
I say keep raising the motor until venting becomes a problem

 

[1fishbone]

Re: too much prop slip

Glad you resolved and think what you saved by investigating and not just spending!

Thanks from ALL of us for the learning experience!