Changed prop = extra slip/virtually no bite

Hello everyone,

I previously had a 9 x 9 prop on a 15HP Merc on a 11' tunnel inflatable. I felt the prop did have some good bite and my max speed(via GPS) was 21.9 mph. I then felt I wanted to go with another prop mith more pitch. I purchased a Quicksilver Mercury 9 x 10.5. On the same boat, it would rev out alot right from the start. It took a long time for it to finally catch. My top speed dramatically reduced, which I thought was normal because this time the boat was carrying myself and my dive buddy. We were only freediving, so no tanks and extra ballast.

Last weekend I acquired another boat; 13' 3" Tunnel Inflatable(DUX Inflatables). I mounted the same motor and new prop;9 x 10.5. Last night I took it out for a test ride in the bay. It would not go past 5mph without trying to over rev the motor. As soon as I would throttle up the motor, it would just rev up and I would get absolutely no bite. My trim is set to zero. Here is a picture of how it is set up;



Any thoughts? Now I'm thinking or hoping that the new prop is defective and I'm just turning the hub thru the prop. I won't know till I mark the prop and rerun. but first I'm going to remount the original prop; 9 x 9 and see if I can at least go past 5mph and get somewhat of a bite past half throttle. I'm then going to put back the 9 x 10.5 prop, but this time mark it to see if there is any slipping thru the hub. I'm going to run these test this afternoon after work. And I just want to know anyones thoughts on this matter, as I'm getting really anxious this morning and can't wait to run my test. I will report back with my findings.

Thank you,

Jeff

Re: Changed prop = extra slip/virtually no bite

small motor props dont have much if any cup and are very sensitive to motor height. I raised my 6 hp motor an inch and lost all bite. The 9 pitch probably has some cup to it. Also small boats are very sensitive to weight and loading. If you cant lower the motor you will need to try and find a prop with some more cup or have some added.

Re: Changed prop = extra slip/virtually no bite

a cupped prop will make a world of a difference? So likely the new prop; quicksilver 9 x 10.5 is good. The edges are smooth and there are no cupping. do you think cupping is something I can do myself or have a shop do it?

Thanks,

Jeff

Re: Changed prop = extra slip/virtually no bite

Ive done it myself but dont recommend it, way to easy to ruin a prop or do it wrong. A shop can do it for not a lot of money.

Re: Changed prop = extra slip/virtually no bite

Jeff ...... I race these inflatables, and I can tell you that it is not your prop .... it is your motor setting ..... also ... the motor will cavitate on pull-a-way .... that is how these are designed as they are race boats for top end, not ski boats ....

Re: Changed prop = extra slip/virtually no bite

deejaycee_2000,

Your absolutely right, it is not my prop. Last night I put in another prop and I got the same results. which is nothing but just reving the motor out. MY DUX inflatable claims it is designed for a 15" shaft, which is what I have. My motor setting was set all the way to zero. anything higher it just gets worse. After taking a closer look of what goes on with the outboard when I try to gain speed the outboard raises up a bit and I can clearly see that the cavitation plate is right up the surface of the water resulting in zero bite. My question is does DUX have it wrong with saying a 15" shaft is used to power these boats. Now the motor I'm using is a 15 HP Merc 15" shaft. I don't plan on racing these and I know the capability of this boat. I rode in a buddies with a 40HP tohatsu. and it screams. But what I want to do is just dive off of it. Get to my local kelp bed and be on my way. But I can't even get past 5mph without trying to rev the hell out of the motor. What do you suggest can be done to fix my problem;

.Will cupping a prop work? which is suppose to fix high transom mount motors.

.A Jack Plate?

.get a 20" shaft motor?

.Cut the transom down?

I don't know, I would really appreciate your advice and feedback on this motor.

BTW. here are some pics of the rear of the boat with my 15" shaft. DUX says to put a string in the center of the high jackers and that the prop nut should fall into +/- 1" above or below the string. I think I fall into that spec.