4" pitch drop - 450 rpm rise?

[rosesrfree4u]

Hi! I have an 1988 Envinrude 88 HP 2 stroke (88SPL). My boat is a 17' 1988 glassmaster center console. The boat has only seen trailer freshwater use (clean bottom, no paint) When I bought the boat used 2 years ago it had a 19 pitch michigan wheel vortex and no tachometer. I had an incident with a rock about a month after I got the boat (trolling at very low speed badly bent one blade of the prop) and I decided to reprop rather than repair. Being ignorant, I knew that going up in pitch length would give me more top end so I got a michigan vortex 12.75 X 21.
So I found out last summer that I needed to make sure my engine was getting appropriate WOT rpms!!!! I bought an oem tach and installed it, set the manual trim so that I was almost porpoising and went for a test drive. Boy my face was red when I found out that instead of the 5000-5500 I was supposed to be getting, I was only getting 4350 at WOT 39mph GPS. Everything I've read has led me to believe that going down in pitch about 4 inches would put me in the correct range (~200 rpm per inch). So I bought a new michigan wheel vortex 13.25 X 17 and a new hub (by the way these are all in the same series and are supposed to be appropriate for my engine according to the folks at michigan wheel and they're all aluminum) and went for a spin. The good news, my hole shot was much better, and the boat seems easier to steer if that makes sense, and also much easier to keep at a constant speed and the WOT did go up BUT only to 4800 (36mph). Both of these test drives were in fresh water, calm winds no waves and only myself and 3 gallons of gas on board.
I'm wondering if only seeing a 450 rpm rise in WOT is acceptable or normal? I thought I'd see at least 600 to get me to my WOT min range. I haven't checked the timing or compression since I don't know how yet and thought I should get some opinions on what to do next before spending money I don't really have. The engine was timed and compression tested in 08 when I bought it, and had good even compression on all the cylinders..... I'm not crazy about the idea of going to a 15 inch for just me, there may be salt water in my future, and someday I may want to drag a tube which I'm guessing would mean a 13 or 11(if they even make one that short)!! It was about 82 on the day of the 17inch test compared to 65 degrees on the day of the 21 inch test which i know had some influence, but I have no idea how much. I also haven't changed the plugs yet this year, would that cause any difference in WOT range? Sorry to ask so many questions, I've been reading alot of posts but haven't seen the answer to my main question of if only seeing a 450 rpm rise in WOT for (4" pitch drop) is acceptable or normal?
What kind of drop can I expect when taking the same setup and running it in salt water?

 

[steelespike]

Re: 4" pitch drop - 450 rpm rise?

I don't find a 17 cc only a S-176 16 ft 750 lbs. and a S196 18 ft 1,000.
Its good you stayed with the same Brand/model. As you know the 200 rpm
is a estimate and any number of things could change that. If the boat has a deep V or a less than slick design, If its difficult to get the bow up.if the motor is a little tired or Throttle isn't opening all the way.I think the 17 would be typical for your
motor on similar boats. A 15 would be the exception but wouldn't be unusual.
Its possible if you raise the motor you could gain the missing rpm.
The antivent plate(just above the prop) should be visable at speed.Usually ends up about an inch above the bottom.In the extreme raising the motor could net a couple of mph and some rpm.Probably easily get you to that 600 you were figuring on.

 

[rosesrfree4u]

Re: 4" pitch drop - 450 rpm rise?

Its the S-176, which over time somehow got translated into 17', which is how its registered and so I assumed the correct length but I just went and measured and sure enough its 16'. Guess I can save a little money on registering next year!
Right now the antivent plate is just about dead even with the bottom of the v (hull is a modified v). It does not have power trim or tilt. It looks like the only way to raise the motor is to actually loosen the 4 bolts that mount the engine to the transom, and raise the engine mount (looks like there's about 1 inch of play to adjust up and about the same to adjust down). I'm guessing that's the only way to raise the motor.???

 

[steelespike]

Re: 4" pitch drop - 450 rpm rise?

I have no first hand experience with the 88.Usually the larger motors have a series of holes on the mounting bracket that allow raising the motor.without unbolting from the transom.I have seen a whole motor raised as well what appeared to be about 3".Not saying you need to try 3".If you don't have any venting problems you should be able to raise it to a point just before venting becomes a problem.?With the antivent plate parralell with the bottom of the boat measure with a straight edge along the keel to the antivent plate. Usually end up about an inch above the bottom. Maybe the holes in the bracket will allow moving it up without having to create all new holes.I don't know how well the Vortex will resist venting but being a new prop I would think it would do well.You might try playing with the trim before raising it.If one hole doersn't seem enough but the next seems too much you can buy or make wedges to add or subtract a few degrees.Being a 16' I would think you could get the 17" to the rpm you need.