RPM/Prop/Boatsize/Weight/Type

[mrbarth]

Hello and good morning,<br /><br />I searched and found a lot of topics with RPMs and Probs, but maybe I got it wrong.<br /><br />If I take an engine, let's say like mine 1991 90HP Johnson, with the prop delivered by the shop (I think), there should be a difference in the RPM I can reach if I put this on a Jon boat or on a 24 pontoon (like mine). Or is that wrong ? <br /><br />I can reach just something above 4200 at full trottle. Does this mean I must use a smaller prop or one with "less pitch" to make my engine run faster ? I dont assume that would give me more speed, just more fuel usage ? Or what do I get wrong here ?<br /><br />Thanks for a clarification

 

[roscoe]

Re: RPM/Prop/Boatsize/Weight/Type

Yes, you have got it right.<br />You need a prop with less pitch.<br /><br />Increasing the attainable wot rpm will reduce the amount of stress on your engine. Top speed should increase a bit.<br /><br />It will increase the amount of pushing power.<br /><br />Best fuel consumption will be at approx 3/4 throttle.

 

[WillyBWright]

Re: RPM/Prop/Boatsize/Weight/Type

That's way low for RPMs. Either the tach is wrong, you have a 19 or higher pitch prop, or the motor isn't running right. Or the toons are really loaded up with marine growth. A 90 on a 24' toon should have a prop pitched somewhere between 13 and 17. The 13 would probably be a bit low and the 17 a bit high, depending on a lot of factors.

 

[mrbarth]

Re: RPM/Prop/Boatsize/Weight/Type

Thank you,<br /><br />The toons are clean, I've got the boat since 6 months, and I assume the preowner "fixed" it several times without real knowhow. I'll buy something between 13 and 17 today. As I saw in the forum, it never hurts to have more then one prop.

 

[walleyehed]

Re: RPM/Prop/Boatsize/Weight/Type

I agree with willy, but I believe I would error on the low side of 17 and go with a 13 to start, and possibly a 15 at max. I don't think you'll get the R's you need with a 17.

 

[mrbarth]

Re: RPM/Prop/Boatsize/Weight/Type

thanks, I was considering going for 15.<br /><br />just checked the prices for props. If you all have several around, you're sure rich guys <br /><br />Any reason to choose between 13*15 or 13 3/4*15 ?

 

[Solittle]

Re: RPM/Prop/Boatsize/Weight/Type

Your best bet is to contact a prop shop and describe your situation and ask for recommendations. Select the one that seems to make the most sense to you and ask them if you can swap for a different one if the first recommendation does not work out.<br /><br />The guys are correct in telling you that you are way over propped. Think of it this way. If you have your 90 on a 12 foot hydroplane and you get 6000 RPM flat out (with whatever prop at whatever speed). Then you take the motor/prop and drop it on the back of a 40' barge. Would you expect the same RPM?

 

[mrbarth]

Re: RPM/Prop/Boatsize/Weight/Type

SoLittle, it makes sense of course to see it that way.<br /><br />Strangely enough I had this image in my head, that the engine would run with the same rpms where ever it is attached to, but moves the object slower or faster, depending on weight, water displacement and form. Something like if I am in the water with fins on and push a buoy or a boat. Although I flip up and down with the same frequenz (rpm) and power (hp), the boat would be slower. Still is in my head though <br /><br />Have to think about what I think wrong. <br /><br /><br />TO find my pitch I searched the board and there was some discussion and methods in Nov. 2003 and Jan 2004, but nothing concrete.<br /><br />The only digits I find are H8362 on the shaft near the end (aft) of the prop. Can I derive anything out of that ??<br /><br /><br />Thanks for the advice

 

[walleyehed]

Re: RPM/Prop/Boatsize/Weight/Type

There may be a # cast into the hub aft end, next to the prop nut.

 

[mrbarth]

Re: RPM/Prop/Boatsize/Weight/Type

Thanks walleyehed,<br /><br />the H8362 is on the outside at the end (near the nut) I checked the other parts put can't find anything. But the prop is bent at one blade, so it might be a good idea to get a new one anyway

 

[walleyehed]

Re: RPM/Prop/Boatsize/Weight/Type

There could also possibly be small numbers stamped on the sides between 2 blades as well.<br />A good prop shop can fix minor bends with no problem....as long as no more than 1/3 of the blade is missing, for about half the cost of a new one, and they can tell ya what pitch you have as well.

 

[mrbarth]

Re: RPM/Prop/Boatsize/Weight/Type

thanks walleyehead,<br /><br />I'll get the prop off this week and try to find a prop shop.