Looking for More efficient/powerful? Prop.

[towtruck1]

Hello All

I'm the custodian of a Calibogie 15 (15') with a Tohatsu 50 2-stroke. It currently has a standard Tohatsu 3-blade aluminum prop.

I use it for recreational skiing and wish I could get just a tad more speed out of it when I take it to a lake at about 5000' elevation once a year (It' fine at sea level). Would switching to a specialty prop of some sort (SS?) help?

Does anyone have any suggestions for getting a tad more out of the engine itself? I don't need any more power out of the hole, although I can't really afford to give up any there.

Thanks to all for any suggestions or comments.

 

[QC]

Re: Looking for More efficient/powerful? Prop.

Well you can very rarely have your cake and eat it too, but there may be some room depending on what the performance is now. We need dead nuts accurate RPM at wide open throttle, trimmed and loaded as you will typically run her. Along with that we could use GPS verified speed. With this info (accurate) we can determine if the current prop is giving you all of the engine and boat combo's potential . . . Also, you will need two props to get what you want . . . maybe three if we don't like the current one for either place you run her. There is no way to get a prop that will perform well at sea level and at 5000 feet . . . My guess is at least two and maybe four inches of pitch between the two. Yes, SS can make a big difference, especially in top speed. Aluminums of the same pitch can actually be better for hole shot though.

 

[towtruck1]

Re: Looking for More efficient/powerful? Prop.

Thank you QC

I'm perfectly happy to get a new prop for the lake, and use the one I have for down here.

Would you need RPP's & speed etc. for the lake, or can one project what I need based on RPM & speed down here?

I understand how an aluminum prop will flex more out of the hole & give me more RPM & therefore power, but I don't understand how a SS prop helps with top end, assuming they both have the same pitch under top-end load. Knowing this might help me.

So where do I go from here?

Thank you!

 

[QC]

Re: Looking for More efficient/powerful? Prop.

You would benefit with proper jetting for 5000 feet too, check in at the appropriate outboard repair section below; Dive Captain, dive . . .

Two answers to your SS vs. Aluminum question: 1) SS blades are thinner so they are more efficient through the water and 2) the Aluminum flexes at top speed too, so the effective pitch is lower . . . Shows up as higher slip % although it technically isn't . . .

The right way to do this is to get us the numbers I mentioned above at 5000 ft. . . . Since you probably can't you may also ask about the associated rule of thumb in the repair forum and/or hope that someone with experience will chime in here. I just don't have any personal experience with that size of a motor although I do have some with bigger I/Os. I needed 2 inches less at 3000 ft. recently and 4 inches at 6500 ft. Different animals for sure though.

I just found a decent listing of operators manuals online for Tohatsu. There is a good listing of propellers and there may even be some jetting info. I didn't dig too deep. Need to know 2 or 3 cylinder . . .

 

[towtruck1]

Re: Looking for More efficient/powerful? Prop.

Thank you!

I'll look into a SS prop and check out the Ouboards forum to see what I can learn about appropriate carb jets for 5000', and performance in general. The owners manual is silent about jetting/high altitude operation.

You mentioned 2 & 4 inches less... I assume you meant prop diameter? That would give you more RPM; but how would the lesser blade area affect power out of the hole or speed? Wouldn't you get more slippage?

 

[Silvertip]

Re: Looking for More efficient/powerful? Prop.

No -- QC was talking about 2-4 inches less pitch. Diameter will take care of itself when you select the pitch. Pitch is measured in inches, not degrees as many think. Less pitch = higher engine rpm at any given speed. More pitch = less speed at any given rpm.

 

[stubblejumper]

Re: Looking for More efficient/powerful? Prop.

More pitch = less speed at any given rpm.

 

[QC]

Re: Looking for More efficient/powerful? Prop.

He meant More pitch = less RPM at any given speed, or higher speed at any given RPM assuming you have enough horsepower to swing it, which is a big assumption . . . Often noobs come on here and suggest they will automatically get more speed by going up 2 inches in pitch. If that was true then why not go up 10 inches in pitch and go 100 MPH . . . What you have to do is match pitch to horsepower and load. More load you need less pitch to get the engine RPM up to it's highest output RPM, less horsepower (altitude eats horsepower) you need less pitch to allow the engine to reach rated RPM . . .

 

[towtruck1]

Re: Looking for More efficient/powerful? Prop.

OK

That all makes sense to me. At some point, it seems there is going to be some crossover between any speed I may gain with higher RPM (limited by engine-manufacturer recommendations of course), and any speed I will lose due to lesser pitch. So I have to figure out what that sweet-spot is. Basic prop charts are there (my Tohatsu manual has one), but I think I really want tweak it to get the max.

Any suggestions for articles/links/charts etc. That would help? I suppose I have to start with a known quantity (how fast the boat goes now at what RPM with what prop) and tweak to get where I want to be.

 

[QC]

Re: Looking for More efficient/powerful? Prop.

I suppose I have to start with a known quantity (how fast the boat goes now at what RPM with what prop) and tweak to get where I want to be.

Eggzachary . . .

 

[towtruck1]

Re: Looking for More efficient/powerful? Prop.

Thanks

Looked at the prop last night. Measured about 5 & 3/4 from center of hub to tip of blade, so call it an 11.5" diameter. A stamp on one blade says 'D 13.5', so it sounds like a 13.5" pitch. No clue what the D means, unless it refers to the hub or engine type , as in M50D.

I thought it was a Tohatsu prop, but if I look at their prop chart for the M50D I don't see any half-pitches and the closest diameter is 11.4. Combined, the closest match is probably 11.1 x 14.
http://www.tohatsu.com/accessories/prop.html

Did I interpret the information correctly? I can't find any prop by that description on the web. If anyone can tell me what prop it looks like I have, or what it appears to be optimized for, I would appreciate it.

Add'l info:
from: http://www.iboats.com/Calibogie_Boats__Calibogie_15__1991/bp/64b108951r1

The 1991, Calibogie 15 is a 15.17 foot outboard boat... The [empty] weight of the boat is 550 pounds... (I assume that is w/o engine as well).

P.S.
It occurs to me that if I read my scale wrong and the prop really measures 6.75 from center to tip, I would have a 13.5" diameter, and that might explain the "D"... So how would I know what the pitch was?? (The prop has not been re-pitched or worked at all since new.)