Engine reving up on turns and straight running

[exceed]

Re: Engine reving up on turns and straight running

I like the jack plate idea it avoids possible damage to the structure of the boat
and provides an excellent device to fine tune motor setup.
Nice that they owned up without any tap dancing,try this,try that,etc.

Guess you don't own a carolina skiff.



Scaley, If you need any other information about your carolina skiff come to carolinaskiffowner.com we will try to help you.

 

[Joshua Nichols]

Re: Engine reving up on turns and straight running

Tell them it must be a powered model...

For sure...

 

[scaley]

Re: Engine reving up on turns and straight running

Got the Jack plate on and adjusted so cavitation plate is 3/4" below bottom of the boat. Just got back from trying it out. No cavitating at any time. Comes out of the hole great trims out very nice. Carolina Skiff resolved this with no hassle.
Also took it WOT and only could get 4500 rpms max. Speed was 35.5 mph. This is with a 3X14X21 Prop. This is with an average load. seems like it should run at higher rpms. It's supose to run between 5000 and 6000 rpms. Thanks for all the replies you have been a big help.

 

[steelespike]

Re: Engine reving up on turns and straight running

The 21' is pretty far up the list.
Does the boat respond to trim up?Are you able to trim all the way?How is the hole shot?
Is the motor fully broken in? Is the throttle opening all the way at the motor?
Don't assume because it is new that its right.
To get to 6,000 you need to drop to about a 16" prop.(It wouldn't seem the dealer would put a prop that was 5" too big)About 200 rpm per inch.=
about 5,500.
If your having no venting issues the motor might be a little low.
As long as you have the jack plate raise the motor to a point just before venting becomes a problem.If you could gain an inch it might get a couple of 100 rpm. and a little speed.

 

[scaley]

Re: Engine reving up on turns and straight running

When coming out of the hole I have the motor trimmed all the way down and it comes up very quickly. I can barely trim it up when on plane. It starts to vent when trying to trim it up. Could this be do to the prop size. The motor is not fully broken in yet but it can be taken to WOT for short periods. I will check to see if the throttle is opening up fully. I"m not to concerned with more speed as 35 mph is not bad. I would just like to get the motor to do what is suppose to and get the trim to be more adjustable.

 

[steelespike]

Re: Engine reving up on turns and straight running

Can't be sure about a prop change unless we can get the boat to the best setup and speed.It appears the motor is still high.If you can't get good effective trim the boat may not be at max speed and rpm.We need to know the actual rpm and speed the motor can make in order to figure what prop is best.There are props that may
work at the present height but without accurates rpm and speed its difficult to know what pitch to select.

 

[scaley]

Re: Engine reving up on turns and straight running

Okay per your advice I did some adjusting with the motor height it was still too high. I lowered it little more then took it out and ran it. It now comes out of the hole better and I have all the trim I need plus. It handles great now. Glad that is finally done. I ended up with the cavitation plate 1" below the bottom of the boat. I believe it's right now. Now I need to get the rpms where they should be. With the 21" prop at WOT I'm running 34.2 mph at 4400 rpms. Can you recommend a prop that will put me where the rpms need to be . I''m sure the dealer will swap out props with me to get it right.
Your help has been tremendous .

 

[bear_69cuda]

Re: Engine reving up on turns and straight running

If WOT rpms are correct (tach is accurate) and currently 4400 with 21"P, each inch P you subtract = ~ 200 rpm increase, in theory a 13"P prop should reach 6000 rpm. Not sure with aluminum, someone will correct me I'm sure...

If this is true the hole-shot should be incredible compared to the 21"P

 

[madgadget]

Re: Engine reving up on turns and straight running

21 to 13 is some drop.. but then 4400 rpm is way low. It will be interesting to see what this gains, I bet its quiet a lot when setup right.

 

[barbosam]

Re: Engine reving up on turns and straight running

It sounds to me that your problem is more likely cavitation and not ventilation. The fact that the anti ventialtion plate is below the keel and your still having issues is a big red flag that your problem may not be ventilation. I've never installed a suzuki but I do speak to evinrude and yamaha application engineers on a fairly regular basis and they always stress that the anti-ventilation plate should never be below the keel; it should be in line or up to 1 inch higher. The higher the engine is mounted the more efficiently it will operate, as long as its not ventilating. I'm sure the suzuki engineers would have the same advice. (I certainly don't represent any of these manufacturers, I'm just relaying info from my conversations with them)

The next red flag is the extremely low rpms. This tells me that the prop is experiencing such excessive loads that it will not allow the engine to come anywhere near its correct operating rpm. These extreme pressures on the face of the blade can actually cause the water to "implode", or cavitate, and this is when the engine will suddeny start reving as if it was "ventilating".

First thing I would do is to inspect the face of the prop very carefully. If you notice very small pitts in the surface of the metal, then I would say you are definately cavitating. If you don't see them, it may be due to the fact that you have very few hours on the engine/prop and doesn't necessarily mean that there is no cavitation occuring.

I would then try switching the prop out but I would not step all the way down to a 13" pitch. I would start at 17" and see what happens. If the problem is resolved, I would then bring the motor back up untill it is at least inline with the keel. Then, if needed you can try a lower pitch prop to dial in your WOT rpm.

Prop issues can be sort of mysterious black magic, although there is some science behind it, when it comes down to it you just have to experiment.

Good luck and keep us posted, I hope that the info I have provided can help.

and someone please correct me if I mis-stated something, there's always more to learn!

 

[steelespike]

Re: Engine reving up on turns and straight running

This is a new boat,new motor setup.The factory admitted to him the transom is too high and installed a jack plate.They also reccomend about 3/4" below the bottom.
He appears to now have the motor at an ideal height. The gear ratio seems to be numerically high at I think 2.69:1 this would seem to make the 21" prop seem a little more reasonable. Though it appears it is indeed too high. It also seems extreme to go to a 13" pitch though it adds up.Why would the dealer install a prop that appears to be 8" off the correct pitch.I think cavitation would result in a higher tach reading and speed that is poor and doesn't add up.
I would still like him to check the throttle advance and confirm the tach is right.
It could be on the wrong setting.
If everything is right going to a 13 from a 21 would be like going from a Honda civic to a fuely dragster! Something is screwy.

 

[barbosam]

Re: Engine reving up on turns and straight running

I agree steel, it would be a good idea to check the tach and the throttle advance to make sure you are getting correct information and that the motor is running properly. That is strange that they would recommend mounting the engine that far below the bottom; must be due to the flat bottom on that hull. And I have seen cavitation be intermitent as he was describing, sometimes trim can induce it, sometimes an obstruction on the hull bottom. But either way if he checks the two things you suggested and they are ok, then its definatley time for a prop change.

 

[scaley]

Re: Engine reving up on turns and straight running

Okay, Now that we have figured out that the throttle and the tach are were both working . So the dealer sent me a 13.5 x 15" prop to replace the 14X21". I put it on and headed for the water. This made a big change. Now at WOT the rpms are at maxed out at 5,900 rpms. Which I consider to be great. Whole shot now is very good. Although I dropped 3 mph down to 32 mph (GPS) at 5,900 rpms. The boat responds to trim and steering much better. The new prop is a Solas Amita 3.
I may now experiment with the motor heigth by raising it up a little to see if it will increase speed a little. Overall this has been quite a learning session for me. I do have to give credit to the help I have received on this forum. Also the dealer whom I bought this from have done all that I have asked as well as Carolina Skiff.
Now if I can ask for a little more help. I would like to go to a stainless prop. Can you give some advise on which to go to ,maybe even a four blade?

 

[steelespike]

Re: Engine reving up on turns and straight running

Without being too specific a 4 blade will allow the most height with te least venting issues.Also as I've said before better handling.Unfortunately a 4 blade can sometimes reduce top end.A ss prop should give overall best performance though it could vary from a few tenths to a few mph.at a cost of 2 to 3 times as much.
You might consult with the guys at the Carolina Skiff forum I deleted the address
but they were helpful in determining if your boat had a transom height problem.
It does appear from the little I read there that the boat can be sensitive to added weight, passengers.