58 Fastwin 18 prop question

[ouachita57]

So I converted my 58 fastwin to a pulse type fuel pump and got it to running good. i mounted it to the back of an early 60s 14' aluminum v hull open bow which is probably 200lbs bare hull. I'm hauling probably 200lbs of gear and my brother and I are both 6'2" and 250lbs so the motor is pushing around 700lbs plus the weight of the motor. It pulls fine but its a little slower than I want(about 15mph) If I change the prop pitch to give me more speed do you think it will be too much for the engine? I'm not sure of the pitch but it appears to be the factory prop. Thanks

 

[kfa4303]

Re: 58 Fastwin 18 prop question

HI ouitchita57. Before you change the prop, you may want to experiment with the engine height. These old OMC motors have sort of an odd leg length of about 18" rather than the standard 15" for a shrot shaft and the 20" for a long shaft. As a result, the leg tneds to hang down too deep in the water on most standard transoms. You'd like the anti-vent plate found just above the prop to be level, or slightly higher (1"-2") than the keel. There are several tricks you can try to get a better position. The one I did was to simply clamp the motor higher up on the transom. It's won't raise all the way, but you should be able to raise it a few inches, which should help. You can also buy/build a jackplate to get the motor to the perfect height. I get about 20 mph using a '66 20 hp on a 14' semi-v hull. Here are some pics.

 

[F_R]

Re: 58 Fastwin 18 prop question

So I converted my 58 fastwin to a pulse type fuel pump and got it to running good. i mounted it to the back of an early 60s 14' aluminum v hull open bow which is probably 200lbs bare hull. I'm hauling probably 200lbs of gear and my brother and I are both 6'2" and 250lbs so the motor is pushing around 700lbs plus the weight of the motor. It pulls fine but its a little slower than I want(about 15mph) If I change the prop pitch to give me more speed do you think it will be too much for the engine? I'm not sure of the pitch but it appears to be the factory prop. Thanks

So you are considering a prop with more pitch than the factory stock prop, right? You WILL be disappointed.

 

[ouachita57]

Re: 58 Fastwin 18 prop question

Thanks for the advice..I'll try playing with the engine height. I didnt know with it being a small hp engine if a prop change would have a big effect of the performance..

 

[ouachita57]

Re: 58 Fastwin 18 prop question

I jacked the motor up 2.5 inches and dropped some weight and got up to 23 mph and spun the hub in the prop.. So now I'll have to wait on a new prop and do some more testing..

 

[kfa4303]

Re: 58 Fastwin 18 prop question

23 mph is pretty good. That's an 8 mph increase (50% faster than original 15 mph), not too shabby. Although, I guess you still have to get a new prop. It should be handle to handle the new found warp speeds a little better.

 

[Brokeback Johnson]

Re: 58 Fastwin 18 prop question

Try a 9x9 prop to wake her up. 9x10 will make the engine sluggish with those kind of loads.

 

[ouachita57]

Re: 58 Fastwin 18 prop question

How much difference would the 9x9 prop make on the top end speed?

 

[steelespike]

Re: 58 Fastwin 18 prop question

The motor new was rated for about 27 mph on a light reasonably fast boat.Came with an 11" prop.
Motor is intended for a bare 14-16 ft boat.I think you need to evaluate the motor lightly loaded.
This should help pin down the overall setup and evaluate the performance of the motor.
And you do need both rpm and speed to do it right.23 with your load may be pretty good.
On a different and heavier boat with a shallow v bottom I did get to plane with 5 adults aboard.
This was the exception but is an indication what may be possible. this same boat also occasionally made an indicated 26 mph
lightly loaded.Do we know what prop we have now? Rpm and speed will indicate how well its working.
Tinytach will get the rpm.

 

[ouachita57]

Re: 58 Fastwin 18 prop question

The engine needs to be raised another 1.5 inches to be the right height.. Im not sure what pitch it has now but it appears to be the original one.. To reach 23mph I removed 2 deep cycle batteries and the rear casting deck.. Im not sure of the rpm as i havent got a tach yet

 

[kfa4303]

Re: 58 Fastwin 18 prop question

The prop dimensions are usually stamped on the face of the prop itself right behind the prop nut. It may have been painted over, but it should be there.

 

[steelespike]

Re: 58 Fastwin 18 prop question

The 14ft Will struggle more with a load than a 16 or even an 18'bare bones utility.
There are small 14 that likely measure slightly under 14 and are shallower and narrower.
What I call a big 14 will be 14ft or more wider and deep and usually rated for higher horsepower.
These bigger boats handle weight better and get up on the water better with a load.
These older motors the numbers are usually stamped near the base of the blades.

 

[scuba_redneck]

Re: 58 Fastwin 18 prop question

I have a '73 18 hp that I have been doing some work on here lately and have got it almost right. I have it mounted on a wide 14' starcraft v-hull with a wood bottom over the aluminum hull. I do not know the exact dry weight but I do know with 6 gallons of fuel, 1 deep cycle battery (about 50 lbs), the motor's weight, and my 200 pound carcass sitting in the back I can get 21 mph with a 9x10 prop. If I add another 200 pounds into the boat no matter how I adjust it my top end speed drops off to 15 mph. The 9x10 prop is just not going to give you the right rpm/ power range that you want. If you would like I have put together an excel spreadsheet that will give you the theoretical speed that a prop should give you at no slip and I have also calculated a 5% slip factor in. I do not know how yours is geared, however mine with a 9x10 prop should give me a max speed at WOT of 4500 rpms of 24.35 mph. The full list of prop options include
9x11= 26.79 mph
9x10= 24.35 mph
9x9= 21.92 mph
9x7= 17.05 mph

These speeds assume that there is no slip or other power loss at my WOT rpm of 4500 rpms. Your motor may be geared differently. Mine has a gear ratio in the LU of 12:21 ie 12 rotations of the prop shaft for every 21 rotations of the flywheel. I can email you the excel sheet if you would like. You can adjust the gear ratios, prop pitch, etc. to fit your motor.

The lower pitched props will allow you to handle more load at a reduced speed. The key issue is to prop your boat so that your engine is able to be close to it's rated RPM range at a given load.

 

[ouachita57]

Re: 58 Fastwin 18 prop question

To give a little info on my boat it is exactly 13'5" long and 57" wide at the middle and narrows to 52" wide at the back.it has a sharp v in the front and is a flater v in the back.. I dont know the exact year of the boat but its probably late 50s early 60s. Its thick aluminum with 3 foam filled bulkheads(seats), 2 men cant pick it up.. The prop is 9X11.. I had 2 casting decks, 2 deep cycle batteries, 50lbs of tackle and 2 250lbs men.. I took out the rear casting deck the batteries and the tackle. and gained 8mph from 15mph to 23mph..I'll try to get pictures.. I'll pm you my email address for the excel sheet.. Thanks

 

[ouachita57]

Re: 58 Fastwin 18 prop question

I'm going to do away with the green paint and polish the hull when I get everything dialed in..