New Prop - Give a little, get a little

Danno · 09-24-2006, 06:45 PM

So I swapped out my dinged up 3 x 13.75 x 17 prop with a new 3 x 13.75 x 15 prop. I needed a backup so I thought I go down in pitch. Here are the differences:

17 pitch prop
- Hole shot stalls my 90hp Honda/19' Arima
- top speed about 35mph (GPS) and top RPM of 5200.
- I suspect I'd get better mileage with this prop.

15 pitch prop
- Hole shot takes off like a rocket, no stalling
- Top speed of 32 mph at 6000 RPM (Honda's recommend WOT).
- Engine whines a lot more above 5200 rpm.
- Engine will go up to at least 6200 rpm but I didn't want to push it.

Both runs were with a light load but 35 gal of gas on board. I really liked how the 15 would allow me to get up and go but that 1000RPM difference seems a bit much.

I think I'd like to find a 16 pitch prop but neither Honda or Merc seem to make one. Make Sportcraft just didn't have one. Apparently, the older Hondas (1998) have Merc lower ends. So are 16's a custom size? How much are custom aluminum props?

STGRule · 09-24-2006, 08:00 PM

Maybe get the 15" cupped?

Mr_sturgeon · 09-26-2006, 12:23 AM

cupped made a world of difference on my boat, 350 volvo penta w/15" stock was pushing 28NM at 4300 rpm, had it cupped and now getting 35NM at 4000
just my experience
good luck

this was on a heavy 26' fiberform

Quiet Riot · 09-27-2006, 09:45 PM

Was the 17p prop cupped and the 15 not? Normally you don't see a 1000 rpm difference with a 2" pitch change. Trim angles the same? Is the 15p cavitating?

A cupped 15" will lower your r's a little, also look at the possibility of diff diameter props. And most prop places like seattle prop, michigan wheel, etc can change the pitch +/- 2" if you send it to them. There is probably someone local around here at a prop shop that can do it or have it sent out also.

Danno · 09-28-2006, 09:18 AM

I just recently purchased the boat so I don't know if the 17" was cupped. The previous owner kept a lot of his past receipts and I didn't see anything like that among them. The trim angles were the same although I found that the 17" would tune into a very tight trim range for max speed whereas the 15 had a wider range (but same basic trim). In other words, a slight change in trim would drop the speed by 2 mph on the 17 but hardly affect the 15".

The 15" cavitiates really easy if I try to accelerate too much out of a turn but it's well within my expectations. In straight line accelrations, it's fine. I like the thought of having the 15" cupped. Any price range on that? I think there's a well respected prop place out of Clackamas based on other's posts here on iFish. I'll have to look them up.

Catching Nemo · 10-03-2006, 08:14 PM

I would forget the 15, as it is using more gas and providing less MPH. If it is aluminum, I would check for cavitation burns in the paint and eventually in the metal (it will start to develop little pinholes).

If a better hole shot is what you want - go back to the 17 and put on trim tabs. Smart Tabs, from Nauticus, did wonders for my boat (better hole shot, on plane in half the time, and increased speed). Made my 130 Honda feel like a new and more powerful engine. Plus you can easily get them for less than $200. GI Joe's often sells them, for $99, on special clearance.

Chrome Bumper · 10-04-2006, 08:58 AM

When you decrease pitch you want to increase area (diameter) to maintain the same mean pressure, so the tendancy to cavitate doesn't increase.

Danno · 10-04-2006, 03:26 PM

Nemo, I'm not looking so much for the hole shot but better acceleration when needed (ie: loaded down). I will keep an eye out for the cavition burns. I never knew that was a potential problem until I saw a photo of a prop here on iFish that was described just as you noted.

Chromie, I checked my outboard and I have less than an inch of clearence so I don't think I can increase the diameter at all.

Chrome Bumper · 10-04-2006, 03:57 PM

Is you engine mounted up a hole on the transom mount? Moving the engine lower will also reduce cavitation. Have you considered a doelfin type thingy. I got an engine that had them, reduces top end so I left them off, they do reduce cavitation and low speed wake and help it get on plane.

4 blade props are avaiable.

I haven't seen 16" pitch in most props, repitching the 15 or 17 to 16 is doable, I wonder if you got a bad 15 incher right out of the box?

A little cavitation burn is no biggy, about the time it gets bad you nick or bend the prop on debris or wear it out in sand and have to get a new one anyway.

Ampro lets you try before you buy I think.

I hope the prop in the mail for me from iboats.com will be an improvement. Hoping for a little better hole shot and topspeed.

5 Salt · 10-09-2006, 11:43 PM

Go with a stainless prop for best performance. Less flex and cupped design the maintains shape under load will give you best perfromance.

16 in stainles might just be the perfect prop by your description.

Danno · 10-10-2006, 12:26 PM

How sweet would a SS prop be! I am trying to get out of the "upgrade the boat, spending lots of money, and beg for forgiveness" mode with SWMBO so the SS prop is on my loooong term purchase list. Besides, being a rookie boat owner, I figured I'd shred an inexpensive prop or two first.

toblerone · 10-18-2006, 03:20 PM

I have a 14.5" 17 pitch and a 14.25" and at 19 pitch. I just switched to a 14.5" 17 pitch stainless and picked up noticable hole shot improvement and top end of about 4mph at 3900 rpm. picked it up on ebay for $90 plus 13 for shipping. A $320. value. Worth taking a look on line.

John