Johnson 90hp slipping in turns

[Clint37]

Ahoy!<br />I just had a 2000 90hp Johnson with low hours installed on my 17ft Astro. (The original 1992 115hp Mercury's crankshaft broke so I opted for a newer engine rather than a repair that would have cost almost as much) I'm somewhat dissapointed in the 10mph loss in top end speed. (from 52 to 42mph) partly because the owner of the Marina that sold me the motor said it would probably run as fast as the older, well worn 115hp. I am also disappointed that I can't turn the boat at WOT without it slipping badly. The Marina lowered the motor an extra notch on the transom which helped but it still slips more than it should. I had a stainless steel prop and Dol-Fin on the Merc but the newer Johnson has neither. It has a 19 pitch alumininum prop with a few nicks around the edges. At WOT, I'm running about 5200RPMs. Isn't this kinda low? The tach was installed with the motor so I feel it is relatively accurate. Any suggestions on solving the slipping problem or prop/rpm suggestions would be greatly appreciated

 

[Spidybot]

Re: Johnson 90hp slipping in turns

You are pointing some of the obvious things out, yourself.<br /><br />Alu and SS props doesn't perform the same and depending on the way you use the boat, you may need the special features that some SS's will deliver. Better grip, ability to work on an air/water mix (surfacing) and more lift, for some.<br /><br />Your experience shows, that your previous Merc-prop and the current Johnson-prop are way different.<br /><br />Lowering the motor is a way to solve the 'slip' (ventilation when air is drawn to the prop) but not normally what you want to do as it adds to the drag from putting more into the water behind the boat.<br /><br />I'd suggest you start out with the antiventilation plate in approx. level with the keel line and then get a prop that will perform with it.<br /><br />Your max rpm at wot is stated 4500-5500 so 5200 is OK. If you raise the motor on the transom and/or switch to another prop design you may get different rpm with the same pitch so be prepared to do some experiments and tests.<br /><br />If you like, you can get the booklet about props via link on my homepage.

 

[Clint37]

Re: Johnson 90hp slipping in turns

UU,<br />Thank you kindly for your valuable information! I will indeed check out your website. By the way, would a Dol-Fin type device enhance the antiventilation plate's effectiveness? Thank you again!

 

[Spidybot]

Re: Johnson 90hp slipping in turns

Uhm - you certainly would benefit from that book <br /><br />The A/V-plate is the plate above the prop. It's there for more reasons, most important to keep air from getting sucked into the prop.<br /><br />It should be approx. level with the keel line and parallel to the water surface when you're on plane (but changes while running when you use trim), as a starting setup.

 

[Clint37]

Re: Johnson 90hp slipping in turns

UU, <br />I just read the book and you're right it answered many of my questions. Thank you for pointing this out to me. I don't know if you saw my edited response so I'll ask again...Do you think a Dol-Fin type device would make the antiventilation plate more effective and cut down on the prop ventilation? Do you recommend any specific brand prop or certain pitch for a Johnson 90hp. The boat is used mainly for fishing/leisure w/only occasional waterskiing. Thank you!

 

[Spidybot]

Re: Johnson 90hp slipping in turns

Fins have been discussed on previous posts - pro et contra. The fins are known to solve certain problems (and to cost some top speed). I'd go pretty far to solve it with a good prop before fitting a fin.<br /><br />As stated I'd make sure the base for setup is right, first. When you know what happens with the basic setup, tweaking it and making notes will tell what effects you get.<br /><br />Then a prop can be suggested. In general, more prop blade area will give better grip (4- or 5-blade props will have more blades producing thrust, even when surfacing). Different cupping and blade design affects it as well.<br /><br />Fill us in on the results from the basic setup.<br /><br />For comparison: I am running a different boat & motor, but my similar problem disappeared and I got all the grip and trim I needed, when I fitted a SS 5-blade.<br /><br />When you look for info, remember iboats' props.

 

[Clint37]

Re: Johnson 90hp slipping in turns

UU<br />THE ANTIVENTILATION PLATE SEEMS TO BE ABOUT LEVEL WITH THE KEEL LINE. IF ANYTHING, THE AVP IS ABOUT AN INCH LOWER. AS I MENTIONED EARLIER, THE OUTER EDGES OF THE PROP HAVE SOME SLIGHT NICKS BUT NOTHING MAJOR. BTW, THE CURRENT PROP IS AN ALUMINUM 14X19. I'D LIKE TO TRY A SS 14X19 AND A SS 14X21. IS IT YOUR EXPERIENCE THAT PROP DEALERS WILL ALLOW YOU TO BRIEFLY TEST DRIVE A PROP BEFORE PURCHASING? THANK YOU

 

[Spidybot]

Re: Johnson 90hp slipping in turns

Yes, a good prop shop will help you to buy only one - that is to let you test several.<br /><br />Describe your situation and they'll suggest a prop to start with.<br /><br />Going up in pitch will reduce hole shot. A 4-blade prop should be tested, IMO.

 

[Clint37]

Re: Johnson 90hp slipping in turns

Well I went to West Marine and Boater's World today while I was in Columbia, SC and looked at their props. All of the associates were very friendly but none of them were certain about which prop would fit or perform the best. Neither of the stores carried any four or five blade props for a 90HP Johnson. I was given a number to Marine Tech which is a company near Columbia that deals almost exclusively in boat props. I'll give them a call this evening. I'm very much looking forward to trying some out. I'll keep you informed of the results. Thank you for all of your help so far!<br />Clint

 

[Spidybot]

Re: Johnson 90hp slipping in turns

It is hard for me to tell which prop will work for you. If you can't find a 4-blade, you may look for a 3-blade with surfacing capabilities.<br /><br />A friend of mine is happy with the adjustable 4-blade composite prop from ProPulse, and even though I've been with him many times I wouldn't be able to transfer experience from his 16" 1860 lbs. semi-flat hull with a 35hp.<br /><br />ProPulse can be found at www.propulse.se <br /><br />US dealer is www.westmarine.com - think model 8902 is right for you and on offer at $128<br />In case of damages spare blades are availavle at $20 and can be replaced separately.<br /><br />You'll see that this prop for your motor is adjustable 16-20" pitch