The prop that we are using now is a 10x8 hill marine on a hombuilt sd. Have had it out a few times and tried several different trim positions. I can't get the RPM's to come up while it is in the water. Out of the water no problem but in H2o it sounds like it is struggling and will only get up to 8 MPH.

I forgot the question. Do you think that a 9x6 would be a better prop for the 18 hp.

That was a Thought. I went over and looked at McMaster and i only have one option wich is a 22 tooth vs the 24 tooth that I am currently running as a top pulley. The bottom pulley is a 30 tooth. Right now I am not sure if the 2 tooth difference will give it enough torque. The 30 tooth on the bottom is as large as I can go. When you first take off the engine gets up to the 8 mph really quick and that is it, cant get it to turn anymore RPMs after that.

Unless you are having some engine problems, your outdrive is not set up correctly or you are trying to push a 24 foot boat, you should have no problem turning that prop. My last 16 duromax build is running a 10X8 with 1.5:1 reduction with no problem. We are getting nearly 22 mph @ 3950-4000 rpms with that set up on a 1436 jon. Post some pics of the set up. Also, what rpm are you hitting out of the water and under load?

16 hp is a long way from 18 hp when you talking small displacement engines

22 mph is also a long way from 8 mph!

I would agree with 50hptiller, even going to the 22 tooth pulley should make a significant improvement.

Out of the water the engine will over rev. I do not have a tac on it but when it is in the water the engine is not even coming close to what it will out of the water. At first i thought that it was the governer so I put a throttle cable on.. It still only reaches the same RPM . Any way I thank you for any suggestions.

Wuts the deal wif the gaff on the end of the fishing rod?

That is just a light weight gaff,it is alot stronger than it looks. Tomorrow i am going to try to adjust the governor on the engine.

Wow! now that is a creation. Like the others said work on your gearing and I think you will do much better. I know you probably only have so many options but that is the reality. I had to change my set-up a couple times before I was happy.

Is that all the negitive trim u have in those pics?

It will go alot lower,I can get the whole prop below the bottom of the boat. I just got back in from adjusting the governor and carb settings. I am thinking that this was the problem. When I was saying that the engine was coming up to RPM out of the water I just didn't know what it was supposed to sound like. It will absolutely scream now. I got a backwater 9x7 in this morning and am going to run them both sat. morning see wich one does the best. It ain't as ugly up close , I just sealed every crack an cranny with 3m gasket. Every thing is riding on stainless bushings and pecision bearings.

Keep in mind that you will not get the same performance out of that 9X7 as you would the 10X8. The 10X8 is a surface piercing prop and the 9X7 is a sub-surface prop. You will have to be able to trim down another 4-5" to keep the new prop from cavitating and add a cavitation plate. Instead of a different prop, I would have spent the money on a tach and a new driver pulley (top pulley).

I am going to run both tommorrow I really think that the 10x8 iwill be keeper now that the governor is set correctly. I will post the results tomorrow.