Prop Question?

[IFish]

I need some help from you propeller professionals. I have a 18' 89 venture with a 94 merc xri 150. It has a four blade prop on it which is not stamp so I can determine what pitch it has. My motor will only turn maximum rpm of 5400. I know that if I drop the pitch down my rpms will go up. I don't know how fast it is going due to speedo not working and no money for gps. However the boats seems to be plenty fast especially when im by myself it is kinda violent ride. Also It comes out of the whole great. I think it has a six inche jack plate which is up to the max.My question is by it only turning 5400 am I really missing out on anything like fuel mileage?

 

[minner]

I believe the max for that motor is 5600rpms so you should be good, check the plate to make sure. If you dropped the 4 blade to a 3 with the same pitch (I know you said you dont know what you have) you'd prolly be right at max. I wouldn't change a thing on a 15y/o motor.

 

[wrechin2]

That venture would love a 27 OMC SRX prop. It is one of the best props for that boat. I got a friend that has a 88 18' tournament edition with a 175 merc and has the 27 srx. It runs 72 MPH @ 6200. If you set up is not optimized then yes, fuel economy will suffer. There is alot to setting a boat. I ran a modified mecury on a ranger and when me and my buddy took it to the lake we took 7 props with us. Set the jack plate in the middle and ran all 7 props. Raised the engine 1 inch and ran all the props again. All the props that showed speed loss we set aside. then all the ones that showed speed increase we contined to raise up 1/2 at a time and kept a table with all the info wrote down. We then got a rough ideal of what each prop would do and what height it wanted to run then went back to half way and lower 1/2 untill we got a ideal. We fine tuned the 2 fastest props. A 26 chopper and a 23 quicksiler. The quicksilver ran 64.11 gps with 2 thrust washer but the hole shot sucked and the chopper ran 63.84 with a stout hole shot. But fastest prop was 64.11 and teh slowest was 58.36. Nearly a 6 MPH difference. You may find alot of improvement in speed and handling with a different prop. My buddy and I rode in his venture and the thing was solid and flying high. I had one of my 26 chopper with me and we stuck it on. That thing was all over the place. When it started to lift it was crab walking. The bow was moving side to side about 2 feet. It scared me and he finally let off and said he couldn't drive it. Funny thing is the SRX is the OMC version of the chopper. Goes to show how just a little difference can mean alot.

As far as rpms. I like to turn any of my mercs in the mid 6's( dependin on porting if more), It is actually easier on the engine. Better hole shot, faster acceleration, and less likely to cause engine damage if the tempature starts to climb. At 5400 you are "lugging" the engine. It is more prone to detonation and engine damage at that rpm than if turning more. Most mercury v-6 will make good power to 6400 then start going down. Then you are simply turning rpms with no power to back it up. Stock engine powerbands are higher than they are rated for. I have a dyno sheet where a stock 2.4 200 rated for WOT of 5800. The engine actually produces 217 at 6400 then sharply drops to 208 at 6500. So the best place would be at 6400. Some may be 6300.


Just to let you know that this stuff is not written in stone. This is just how I do and never had any issues. Not saying you should. I know alot of people who live by this. I like the HUMMMMMM of a v-6 screaming!!! Cant wait to hear what this 2.0L merc will sound like at 9500! emoEek But the way it is built it has to turn that to get the 295HP out of it. RPMS= hoprsepower. Stock 2.5 200HP 5800, 2.5 225 Pm (.025 higher exhaust port timing of a 200) 6750RPM, 2.5 260hp (.025 exhaust .080 intake over a 200) 7500 RPM, 2.5 330 ( .080 exhaust .120 intake over a 200) Hp race engine 8500RPM. See the relationship between rpms and HP? You can take a 2.5 150 and make any of the above engines with it.

SORRY SO LONG!!! LATER!!

 

[IFish]

James I really appreciate all that info. I like my Venture haven't had it long still trying to figure it out. I wish I new what pitch prop is on it but I have no Ideal since it is not stamped. With two ppl in the boat it balances very well and Occasionally tries to crab walk but nothing serious. However when im by my self it is a brutal crab walk. I to like to hear a merc wind up. The Norris craft I had 17.5' with a freakishly built 200 Merc by Dug Potter with all the goodies you can put in one. Man did that engine sound good at high rpm's. I hope that this 2.5 150 I have last for a good while. But when it's time to rebuild I will probably bring it to you and do a good up grade.

 

[wrechin2]

If it was a 2.4 200 it was limited to what could be done due to the chrome bore. 2.5 can be made into monsters. The thing is a 2.5 150 can be made into a 200, 225, 260, or 330 Hp engine. About 15 Hp less with carbs. I like carbs for the easy of tuning and don't cost a war pention to get the fuel mix correct. People dont realize for the most part the blocks are the same except port timing is what makes the HP. Here is a port map I go by to turn them into 260 blocks (minus the nikasil coating) Jay smith (professional race engine builder in california and forget how many time world champ loves steel bore engines) I am building 2 2.5's that I am porting to 260 specs. So keep a watch for pictures in the future of them. I'm building a baby 2.0L Tuber engine to go on my 350# hydrasport "race Boat" to go eat a few of the big boys up. Got a buddie with a Ally with a modified 225 pro-max (ported to 260). That tuber engine is suppose to make 295HP @ 9500 RPM (coping one that dynoed that)and make a 10K tach a blurr. Should sound good!!!! I should have more HP and 1/2 the weight. If it will not do what I want it to do I will build one that will!!!!!. Just need to find me a skinny driver!!!! </p>

</p>

Like I said the 27 SRX is a good prop on that boat. My buddies boat crabed walked badly with a chopper and was a rock with a SRX. Just have to machine the locknut down to make it work. Just because it is a large pitch don't mean you can't turn it. On my glasstream I borrowed a 21 lazer from a buddy. could only turn it to 5400 rpm at 51 MPH, put my 26 chopper on and could turn it 6000 @55 GPS with 6" setback, 61 GPS with 8" setback, and 68GPS with 10" of setback. I had alot of slipage due to high trim angle to get the boat to lift. When I got it dialed in it was right at 8%. That is why the rpm's never changed just the leverage was better and the prop was bitting better thus my speed was increasing. My 2.0L had unusual low port timing (1.665) and was a blurr to 6K but would hit a brick wall at 6K. My 26 chopper lifted the boat more and I gained RPMS and speed. Very sililar to when you don't trim. Then when you trim you feel the speed pick up and you can watch the rpms climb. Same principle. I have a buddy that had a 18' ranger with a 175 johnson. Had a 21 michigan wheen stainless on it. Pulled a whopping 4600 rpm at 42 MPH. I got out my 24 raker and he said, "if I cant turn a 21 how do you expect it to turn a 24?" I told him to trust me. Put it on the boat lifted so much more and the rpms went to 5400 and 56mp just because it could lift the boat. Just past experiences.</p>