Prop Question 90 suzuki 4 stroke

[hunter066]

I have a 4 stoke 2008 90hp suzuki on a 2008 1860 lowe roughneck alum boat. It weighs 766lb between 1000 to 1100lbs with me and gear in it.I have a 13 7/8x19 solas titan ss prop on it.It runs about 38mph and 5700rpm. I had a 14x19 alum prop on it and it ran the same speed but 5500 rpm. It comes out of the hole great but i would like to get some more top end speed out of it. I would like some suggestions on what size of prop to go up to and how much more mph i will gain thanks.

 

[steelespike]

Re: Prop Question 90 suzuki 4 stroke

Welcome,I can't be too specific but I'll try to get you started.
Do you know the max rated rpm for your motor.I think you may have left your motor out of your total weight.Motor must be over 300 unless your only 100 lbs
I think your rigs a little heavier than you figured.No biggie.
I feel that some jon style moats tend to lay down kind of flat at speed.And can be a little stubborn to get the bow up.A bow lifting prop might help.Its possible the Titan might be I don't know.most motors are setup with the antiventplate ( just above the prop)even with or slightly above the bottom of the boat.If you dont have
any venting(cavitation )issues you might try raising the motor to a point just before venting becomes a problem.Could gain a couple of mph at no cost.
Someone should be along with more specific info.

 

[hunter066]

Re: Prop Question 90 suzuki 4 stroke

I caint find anywhere about how much my motor weighs. Its 08 suzuki 90 4 strokw the boat performs great in everyway the titan does lift my bow. I was just wondering if i could get alittle more speed.

 

[steelespike]

Re: Prop Question 90 suzuki 4 stroke

Motor weighs about 420 (wow)Do you know the max recommended rpm?
Usually they try to prop the 4 strokes near the top of the rpm range.
I assume 5700 is close. If you increase the pitch to a 21.It might get you a little speed but lose a little hole.Probably drop rpm about 300 to 400.

 

[hunter066]

Re: Prop Question 90 suzuki 4 stroke

Motor weighs about 420 (wow)Do you know the max recommended rpm?
Usually they try to prop the 4 strokes near the top of the rpm range.
I assume 5700 is close. If you increase the pitch to a 21.It might get you a little speed but lose a little hole.Probably drop rpm about 300 to 400.

5500rpm is max rpm. If im not going to get more than 5mph than i will just stay were im at.

 

[steelespike]

Re: Prop Question 90 suzuki 4 stroke

I would say 5mph is unlikely based on your present results.If your motor is rated at
5,500 max then you shouldn't run routinely at 5700.You might consider having prop repitched at a prop shop about an inch.Should put you at 5,500 and a little more speed.Raising the motor could also net a couple of mph if you can raise an inch or more.

 

[Silvertip]

Re: Prop Question 90 suzuki 4 stroke

Error #1: You changed prop diameter when you should have changed pitch. Error #2: You probably have a prop from a different manufacturer than the original which is in itself a variable since the two props are different designs. Be aware that diameter and pitch are only two of many design parameters. Error #3: At 5700 rpm, 19P prop and 38 mph that shows 4% prop slip which is verrrrry efficient so I don't have confidence in the numbers. 0% slip means the prop is 100% efficient and for that to happen it would have to be running in a solid which it obviously is not. Error #4: You tried to drop rpm and gain speed at the same time when that is not how this concept works. The long and short of this is that you are probably propped correctly but the setup is not correct. Check the engine height (not trim). The AV plate should be about 1 inch (give or take a bit) above the bottom of the transom. Raising the engine a hole or two may gain you some rpm and speed. Then drop back an inch or two of pitch to reduce the revs but retain the gained speed. When changing prop pitch, decreasing pitch raises engine RPM for the same speed. Increasing pitch lowers engine RPM for the same speed. Less pitch means the engine must run faster to cover the same distance as before. More pitch means it runs slower to cover the same distance. Carried too far however means the engine over revs (too little pitch) or lugs (too much pitch).

 

[hunter066]

Re: Prop Question 90 suzuki 4 stroke

Thanks for the info guys. i will check the height of my av plate.I didnt want to change prop diamater its just what the ss was made in compared to the aluminum. Im sure they are diferent brands. I will check the height of the av plate. I went out and ran my boat today. With just me in it i will run almost 5900rpm at wot stillruns 38mph. If my av height is correct what size prop should i go to to get my rpms back down? Once again thank you guys very much.

 

[Silvertip]

Re: Prop Question 90 suzuki 4 stroke

Error #5: Now you test ran the boat with just you in it. When you add one or two more people the boat now becomes a dog due to the added weight. You MUST establish your base line with what you consider an average load. If you consider just yourself to be an average load then go for it. If not, add the weight and then do the testing. If you prop for a heavier load, that means the engine would likely overrev with only you aboard. However, that's why boats have throttles -- you can throttle back. However, prop for just you in the boat and then add a couple buddies, the engine now lugs which cannot be compensated for with throttle. The rule of thumb is that engine rpm changes about 150 - 200 rpm (up or down) per inch of pitch change. The important factor to consider though is that number assumes the props are from the same manufacturer and same model. Change manufacturers, model, # of blades, and prop material (stainless/aluminum) and all bets are off.