1986 50HP Thunderbolt Propping issue

[Bandchaser]

I have a 86' 50hp tiller that I got in a boat, motor, and trailer combo. The compression, spark, and fuel all tested great. In the barrel, it seems its going to rip your arm off. But under a load, its a real dog. The boat is a aluminum Duracraft 1860.
The motor had a 10 3/8" x 15 pitch prop. Went to a outboard shop, and they let me try a 13 pitch. The boat doubled the speed and almost planed out. When I returned, they said I needed a 9 pitch prop, so they ordered one. Got the prop and installed it friday. Went duck hunting saturday, and the motor is worse now than before.
They ordered a 12.1" x 9 pitch prop. All my research today is telling me that the new prop is incorrect. Please help, as I just don't have the time or the money to be spending on multiple props.

Thanks,
Brian

 

[ckangaroo70]

Re: 1986 50HP Thunderbolt Propping issue

The 13 pitch prop was probably as good as you are going to get for that 1860 aluminum boat with a merc 50 on it. I might be inclined to try a 12 pitch if I were you, but a 9 pitch don't sound right at all to me. I have a 10 3/8 X 13 pitch on my 50hp merc which is mounted on my 16ft aluminum Lowe Hustler. With two large adults and fishing gear I figure I am around 1500lbs loaded. My top speed is around 32 mph and I can get up on plane in a matter of seconds. That 18 footer of yours might actually be a bit under powered with a 50hp. When you say you have an 1860 aluminum boat....something around a 70-90hp pops into my head for good performance. Still you would think you should be able to find a good prop that would allow you some good low in power if you are willing to sacrifice some speed. Kinda like these new Bass Trackers they sell with a 25hp Merc hanging on the back of them. They help keep the overall cost of the boat down, but it won't take long for the owner to be looking for ways to get down the river a bit faster or get up on plane faster.

 

[steelespike]

Re: 1986 50HP Thunderbolt Propping issue

Barrel tests only indicate the motor runs it could be running lean did you try a little choke?
Try squeezing the primer bulb?
I have the 1965 version of that motor on a 57 18' Crestliner Bucaneer utility 330 lbs dry weight.
Haven't had occasion to fine tune the setup or test for actual speed but the boat was setup in 65 by a Knowlegeable
old time boat racer who owned it at the time.It has a 12" prop it has excellernt hole shot top speed is around 35
You very likely have a motor issue. You wouldn't be the first to not realize you were missing a cylinder or perhaps 2.
I also had the 77 version of the same motor on a 77 18' Crestliner voyager utility 375lbs dry weight.It also had excellent hole shot and planed easily with 1,000 lbs of lumber and 2 adults aboard.