Prop problem, 1981 Johnson 115

[Carphunter]

Well, I screwed up my first post and put it under the sailing forum. Let me try again. I have recently purchased a 1981 Dynatrak by Ebbtide, 16-1/2 foot, model #162 fish and ski with a 1981 Johnson V4 115. I am making about 32 mph at top speed. I have been told by many local boat dealers that I should be making at least 45 mph. I have a 4-blade, 12.8 diameter, 17 pitch composite prop. I was told what I needed was a 3-blade, 13-1/4" diameter, 19 pitch stainless steel prop. I tried this prop and was only able to get 25 mph at top speed. My tachometer is broken at the moment, so I am unable to tell exactly how many rpm's I am turning. I can tell that the motor is getting more rpm with the 4-blade prop. I believe that the boat is extremely heavy, and it is a tri-hull. Also, the engine seems to be running great. I have only had the boat for about 2-weeks. Is the prop my problem, or something else?<br /> Thanks in advance for any help!

 

[MGuckin]

Re: Prop problem, 1981 Johnson 115

Motor will rev more with the composite. Lower pitch and the blade will flex much more do to what it is made from.<br />I would fix the tach first. Make sure motor is achieving max rpm @ wot. I believe you will find that it is not.<br />Between hull,motor,fuel,passengers etc., you must have about 2100 lbs loaded.<br />With a 19" 3 blade aluminum, you should be able to do about 45, once everything is set up correctly.<br />HP 115. <br />WEIGHT 2100.<br />BOTTOM FACTOR 190.<br />MPH 44.46239717. <br />MAX RPM 5500.<br />PITCH 18.98284163.<br />But fix the tach first. Experimenting with props gets expensive.

 

[kart33]

Re: Prop problem, 1981 Johnson 115

Start with fixing the Tachometer or replacing it. Outboards are very dependent on RPM and no matter what blade you put on it if you don't know the RPM of the engine you are hunting for an answer that will be inaccurate at best.

 

[Carphunter]

Re: Prop problem, 1981 Johnson 115

Thanks MG, and Cart33. I will replace the tach ASAP. If the engine is not achieving max rpm at wot, what could be the problem? Do you guys have any information on the boat that I own? I tried contacting Ebbtide, and they were not much help.

 

[Spidybot]

Re: Prop problem, 1981 Johnson 115

Carphunter,<br /><br />As soon as the rpm is stated, a number of suggestions will come up. Unfortunately the rpm is crucial so be patient until you have the tach fixed.