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prop matching question
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| Mr T |
Posted on 02/06/09 - 9:47 AM
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My 1970 Nauset is running a 1990 Evinrude 88 SPL motor with a factory 13 3/4 x 15 prop. Motor was rebuilt in 2006, and has about 100 hours on the rebuild The cav plat is level with the bottom of the hull. I have a nissan 6 hp kicker on the back.
With 12 gallons of fuel, and myself, I run WOT about 5600-6000 RPM, with a GPS speed of 33-35 MPH. It seems about that even when loaded for fishing with two guys and all of our gear.
Boat pops up on plane very easily.
I think this is slow for the motor/boat combo, but am not sure.
Anyone have experience with this setup and what prop should be on there?
Thanks guys,
Edited by Mr T on 02/06/09 - 9:48 AM |
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| Tom W Clark |
Posted on 02/06/09 - 10:37 AM
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A 15" pitch propeller is too small for the boat/motor combo. You should be running a 17" OMC/BRP stainless steel prop or perhaps a 15" Stiletto Advantage 4.25.
For OMC/BRP props I recommend the 13-3/8" x 17" (#177035) or 13-7/8" x 17" (#176572) ST or the 13-7/8" x 17" Viper (#176623). The latter two are the same pro but he Viper is polished. There will be no performance difference between the two.
The 13-1/4" x 15" Stiletto Advantage 4.25 is recommended because it tends to perform like a 17" pitch propeller.
Your 88 SPL should be mounted one hole up on the transom, NOT all the way down.
You should see a top speed of about 40-42 MPH, lightly loaded.
Edited by Tom W Clark on 02/06/09 - 10:39 AM |
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| Mr T |
Posted on 02/06/09 - 11:37 AM
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Tom, thanks for the information and even more the part numbers. I had a feeling I was not propped right. 
one thing though, I think I'm gonna go for aluminum over stainless because I run the rivers here in the delta, and it is common to hit things like trees and branches during low water and runoff times. Aluminum will take more of the shock and break if needed over stainless.
Re mounting holes, I'm gonna ask a silly question; the top hole are through the the transom, I can see a set of holes and the motor is on the very top set of them with the bolts coming from the outside and the nuts and washers in the transom well. The bottom bolts appear to bolt to threads on the motor mount bracket, but do not feed through. Am I correct in asssuming the are a series of threaded holes in the motor mount bracket, or do I need to drill more holes in the transom when I raise the motor? 
Edited by Mr T on 02/06/09 - 11:43 AM |
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| Joe Kriz |
Posted on 02/06/09 - 11:49 AM
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It sounds like your motor is mounted using the "blind holes" in the engine bracket.
Yes, the engine bracket has threaded holes for this called blind holes.
This is a very common problem for this vintage of Whalers.
Check out this article and drawing about the blind holes.
http://www.whalercentral.com/articles...icle_id=82
Before raising the engine which will require 2 new lower holes and filling the old blind holes, try a different prop and see if that gets you anywhere close to the top speed you are looking for.
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| Mr T |
Posted on 02/06/09 - 12:07 PM
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Good point. I really am not a fan of more holes in her, she seems very happy the way she is sitting. I'll go with a prop change and see what's what.
Anyone know a good source for alum. props?
Checking the web and fleabay, but if there is a better source, I'm alll for it.
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| Joe Kriz |
Posted on 02/06/09 - 12:53 PM
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Here is the Aluminum prop part number made by OMC for the 17 pitch.
13 1/4 x 17 # 391199 (Aluminum)
The numbers may be superseded so check with your BRP dealer or their online parts catalog.
It sounds like you have the 15 pitch OMC Aluminum prop which is:
13 3/4 x 15 # 391198 (Aluminum)
I had both of the above aluminum props for my 1985 Evinrude 70 hp.
The 15 pitch had a better hole shot and the 17 pitch had a higher top end speed.
Edited by Joe Kriz on 02/06/09 - 12:58 PM |
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| Tom W Clark |
Posted on 02/06/09 - 1:21 PM
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There is a big performance difference between an OMC 70 and an OMC 88/90.
As Joe points out, you will have to fill those two lower holes and drill two new holes to raise an OMC motor that has been mounted all the way down using the "blind holes" but that it the smart thing to do.
The motor is too low as it is and you are loosing performance and fuel economy.
Any new motor in the future will require that those holes get changed anyway so why not do it now and have the flexibility of the BIA standard bolt hole spacing?
Edited by Tom W Clark on 02/06/09 - 1:22 PM |
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| Tom W Clark |
Posted on 02/06/09 - 1:29 PM
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As to the old wives tale about aluminum props protecting your motor, it's not true on modern propellers.
All stainless steel propellers have hubs that give way in the event of a propeller strike. The Flo-Torq hubs even have field-replacable Delrin inserts that you can swap out yourself if they fail.
Any story you hear about a stainless prop causing lower unit damage would have happened with an aluminum prop as well.
Propeller design has come a vry long way int he last 20 years. What was the norm back then is not valid today. You cannnot get the same performance with an alumimum prop.
Aluminum props are still sold now for one reason only: they cost less to purchase.
Edited by Tom W Clark on 02/06/09 - 1:36 PM |
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| breakwaterboatworks |
Posted on 02/06/09 - 8:17 PM
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Not a 100% on Tom's post ,aluminum is still a little more forgiving on prop shafts than SST,but performance out of SST is certainly better as he said.
Eric |
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| ioptfm |
Posted on 02/07/09 - 5:34 AM
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If I'm not mistaken, on the newer props with the Hubs, you can still run at low forward speed, at low rpms, which will let you putt back home in calm waters, whereas if an aluminum prop snaps or breaks, you are dead in the water.
Tom
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| Tom W Clark |
Posted on 02/07/09 - 9:40 AM
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The hubs are the same between aluminum props and stainless steel props. The difference is the stainless steel prop will not get mangled the it time you run it through sand or grave, or hit a piece of driftwood where the aluminum will get mangled.
In fact, stainless steel will last so much longer it is often LESS expensive than aluminum in the long run. Running aluminum props you will taking it back to the prop shop over and over to get it repaired, repairs that would not be needed most of the time with stainless steel.
We could have a theoretical discussion about what "...a little more forgiving on prop shafts." means but why? In reality, you can find good examples of propeller shafts that have been bent. You can also see that it happens more often with stainless steel propellers than aluminum, but that the reason for that is simply that there are far more stainless steel propellers out there. On large outboards, nobody who cares about performance runs aluminum props any more. They are antiquated.
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| Mr T |
Posted on 02/08/09 - 8:31 AM
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Well, it came down to two issues, one was a classic, which is money, the other availability. 300-500 for a prop is outta my range these days.
I bought a rude/brp prop, 13.25x17 p/n 763463. Out the door for about 140, swapped it on, (BTW, thank you to the last person who put the prop on, they DID grease the shaft. Came off nice and easy!)
Went out Saturday to a lake up in the sierras, about 4800 foot elevation, so jetting is not right for it. That said, still popped up easily, a bit longer, but not much, top speed was 36.3 mph, at 4800-5000 rpm. Could not get her to go any higher, seemed the altitude really had her running too rich. With the old prop on that lake, she would pull 5400-5600, although I do not ike running that since she is a almost 20 years old now. (less than 100 hours on a rebuild, but still and all...)
Very smooth, got some chop in the afternoon, and went through it like a champ on plane. Funny how this boat does better going a bit faster compared to other boats I've had....
I'm not too sure about getting 40-42 from this combo though, might have to try rasiing the motor...
Thanks all for the input, will keep ya posted.
Gonne try to get to the delta this weekend, and will post up results at sea level from there.
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| Tom W Clark |
Posted on 02/08/09 - 8:45 AM
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Any time we talk about what should, or should not be expect, performance wise, from a boat/motor combination, it is assumed we are talking about performance at sea level or close to it.
Altitude will have a profound effect on a motor's power. Ceteris paribus, for every 1000 feet of elevation gain there will be a 4.5 percent reduction in horsepower.
You did not mention that you are operating at 4800 feet of elevation. A 90 HP outboard operating at that elevation is only going to produce about 70 HP so the propeller recommendations change to those for a 70 HP motor.
In the case of your boat I would recommend the same propellers but all with two inches less pitch.
At that elevation I would expect your top speed to be 36-38 MPH which is exactly what you report.
The stainless steel Stiletto Advantage propeller I recommended can be bought for less than $250, delivered to your door. It pays to shop around.
Edited by Tom W Clark on 02/08/09 - 8:47 AM |
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| Mr T |
Posted on 02/08/09 - 9:09 AM
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Hey Tom, I don't live at up at elevation, just went fishing there. I'm in the central valley portion of CA, near Sacramento, so the vast majority of the fishing is done at sea level. I know there is a performance drop at elevation which I mentioned.
Thanks for the tip on the stilletto prop, I"ll look for it online. I do have one questions pertaining to that suggestion; you state that the 15 pitch stiletto performs like a 17 is that related to the 4.25 designation you have listed as well?
I looked up stiletto props and they have advantage I in that size but not with a 4.25 identifier. Is there another place to look for that?
Edited by Mr T on 02/09/09 - 7:19 AM |
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| Mr T |
Posted on 03/03/09 - 8:27 AM
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Update on the new prop, got out to th edelta Saturday and went for a spin. With two guys, and 12 gallons of fuel, got to 5500 RPM, showing a GPS speed of 38.4 MPH. Ran like a champ, but still not seeng that elusive 40+. I gotta ask, will raising the motor add that much of a difference? I am going to decarb the motor this week with seafoam, and then swap the plugs, been running pennzoil 2 stroke oil and it smokes like a pig at startup with it. Going back to the XD50 this week.
I have a buddy with a motor hoist in his barn, but it's almost an hour drive to his place from home and no water around there when we finish to go test!
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| DesertSport |
Posted on 03/03/09 - 11:44 AM
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I'm glad to see folks here recommending propping to the higher end of the RPM spectrum. That saves a lot of 2 stroke engines.
2-strokes DO NOT like to lug. Lugging builds carbon and will kill one, in short order.
Not to mention, boats tend to get heavier, with age. We accumulate a bunch of junk that we've forgotten about, and some Whalers tend to absorb some water.
We all think our engines are getting tired, when in fact, they are pushing more weight.
As an example, I have a 2000 Merc. 75 (2-stroke), on a pontoon, that has over 2000 hours on it. It runs like day one. BUT, it is propped to maximize its RPM range. That engine has been WELL maintained and only sees Quicksilver oil and has a good (Racor 10 micron) fuel filter/water seperator on it.
Other than water pumps, tune ups (OEM parts only) and a few hose replacements, it has never been touched.
I'm looking forward to 3K hours.
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| Phil T |
Posted on 03/03/09 - 1:07 PM
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As I have learned from Tom et. al., and from raising my motor, you can expect a gain of ~150-200 rpm's for each hole you go up.
If you run the boat by yourself, no passengers or gear with a full tank, what is the performance? As it is mounted, I would guess 40-41.
1992 Outrage 17 I
2019 E-TEC 90, Viper 17 2+
2018 Load Rite Elite 18280096VT |
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