Need help choosing a prop

[starkyb]

I have a 75 horse Mariner on a 17 foot aluminum Tracker V bottom. The prop on it is a 17 pitch vented stainless. I run about 35 mph at 5600 rpms. According to the specs the recommended rpms should be 4800-5250. I am thinking of going to a 21 pitch prop that would bring my rpms down to the acceptable range. Does anybody know if I would gain or lose top end speed? Any help would be appreciated.

 

[tahoejag]

Re: Need help choosing a prop

From my lessons learned, the higher pitch prop - the lower rpms, i.e. for every inch of pitch is 150 to 200 rpm difference. The lower the pitch, the less top speed. Right here under IBoats.com prop section on the bottom explains a bit about pitch and rpm's, etc. Theoratically - 17p prop will move 17" thru the water - 21p prop will move 21" thru the water per one revolution of prop, hence the gain of top end speed with the 21p over the 17p. But talking props, there are numerous factors in choosing a prop, and the afore mentioned is just touching the surface.

There are alot of very knowledgable persons on this forum that know their sh*t about props and have guided me in the right direction and/or given me more insight than one would have imagined about props. Check out the adult section of the stickies under props to provide detailed info and I'm sure the experts will be by shortly.....

 

[jestor68]

Re: Need help choosing a prop

Plugging those vent holes should help.

 

[tahoejag]

Re: Need help choosing a prop

Plugging those vent holes should help.

Thanks jestor...I myself, with my recent posts about tuning for a prop, totally overlooked that and never mentioned that I am presently running a vented prop :facepalm:. Would have never thought to cover up the holes to see the difference in performance with current prop.

 

[steelespike]

Re: Need help choosing a prop

Plugging the vents might help and is simple enough to do. But it appears your slip now is about 9%. A reasonable
number. I think a 21 would be too much and a 19 should work well. subtracting 300 to 400 rpm.

 

[steelespike]

Re: Need help choosing a prop

Plugging the vents might help and is simple enough to do. But it appears your slip now is about 9%. A reasonable
number. I think a 21 would be too much and a 19 should work well. subtracting 300 to 400 rpm.
A 19" prop at 5300 rpm could produce about 37.2 at 9% slip.10% slip could net about 36.8.
I wouldn't worry about 50 rpm and the 19 could subtract 400 rpm giving 5200 rpm.
The 19 may be slower out of the hole and may be more sensitive to added weight.
A 21 could really kill hole shot .

 

[starkyb]

Re: Need help choosing a prop

That's great info. For a motor that size would an aluminum prop work as well as a stainless one and is there a certain type or brand that you would recommend?

 

[dan02gt]

Re: Need help choosing a prop

starkyb,

The exhaust vent holes are for tuning hole shot and will not change your WOT numbers at all. You're really good slip numbers further back this by showing you're not slipping. The pressure of the water flowing across the barrel of the prop closes them at plain speeds.

Like was mentioned before I think a 21P will be too much and a 19P would be just right. I would look at the Mercury Laser 2. I think you would need the small barrel version with the correct hub kit. They come with Mercury's PVS (performance venting system) that allows you to choose plug sizes to tune your hole shot, and they seem to do well on tin boats.

 

[1FASTLASER]

Re: Need help choosing a prop

starkyb,

The exhaust vent holes are for tuning hole shot and will not change your WOT numbers at all. You're really good slip numbers further back this by showing you're not slipping. The pressure of the water flowing across the barrel of the prop closes them at plain speeds.

Like was mentioned before I think a 21P will be too much and a 19P would be just right. I would look at the Mercury Laser 2. I think you would need the small barrel version with the correct hub kit. They come with Mercury's PVS (performance venting system) that allows you to choose plug sizes to tune your hole shot, and they seem to do well on tin boats.

+1......Very correct information and a GREAT suggestion for a prop It will also probably or could result in a bit higher top end after tuning the PVS holes for holes hot and then adjusting motor height for your hull. The better geometry of the Laser ll prop could b an asset.

 

[starkyb]

Re: Need help choosing a prop

Thanks guys. I will see if I Can find one.