New Build complete - Performance results of first test

[Wave thumper]

Hi,

So my boat is complete, its an 18tf fiberglass fishing boat with 5.0, 4 barrel, 1.83 Alpha 1 and 23 pitch laser SS prop.

The leg is mounted according to the Mercruiser installation instructions at the lowest setting according to heavy duty installation.

At 5000rpm the boat goes 40 miles per hour in a very light bay chop.

It know its not a speed boat, but with my configuration I was expecting a bit more speed out of it.

Especially considering the speed achieved by bigger boats such as Chaparrals.

The boat does not have trim tabs.

I am interested in your thoughts on the performance and if you share my view the speed is somewhat down on where it should be.

 

[NHGuy]

Re: New Build complete - Performance results of first test

Too bad you can't turn back time...deep drives aren't as fast as shallow ones. If you talk the high perf guys they all raise their x dimension, which is another phrase for prop shaft to hull bottom relationship. Depending on your hull design you can run them quite a bit higher than the deepest setting.
But you have what you have. So does it accelerate past 5000 r's? Or does it hold at 5000? I'd think that you could run the next pitch which is 23". That should go a few more MPH. Also do you realize that you can trim the the drive up at speeds above 28 or 30 and lift the bow? This will reduce friction and get you more speed. So give that a shot too.
Since you have a high drive ratio you are going to have to run high pitch props.

 

[Wave thumper]

Re: New Build complete - Performance results of first test

Too bad you can't turn back time...deep drives aren't as fast as shallow ones. If you talk the high perf guys they all raise their x dimension, which is another phrase for prop shaft to hull bottom relationship. Depending on your hull design you can run them quite a bit higher than the deepest setting.
But you have what you have. So does it accelerate past 5000 r's? Or does it hold at 5000? I'd think that you could run the next pitch which is 23". That should go a few more MPH. Also do you realize that you can trim the the drive up at speeds above 28 or 30 and lift the bow? This will reduce friction and get you more speed. So give that a shot too.
Since you have a high drive ratio you are going to have to run high pitch props.

Thanks for your comments.

Yes that speed was achieved with the boat trimmed up as much as possible. Any more trim and the boat will start jumping out of the water.

The boat already has a 23 pitch prop and the cruising performance is good, but I was just expecting a bit more top speed.

Actually the design of the transom did not allow the leg to be fitted any higher anyway so I had no choice here.

 

[Bondo]

Re: New Build complete - Performance results of first test

Thanks for your comments.

Yes that speed was achieved with the boat trimmed up as much as possible. Any more trim and the boat will start jumping out of the water.

The boat already has a 23 pitch prop and the cruising performance is good, but I was just expecting a bit more top speed.

Actually the design of the transom did not allow the leg to be fitted any higher anyway so I had no choice here.

Ayuh,.... Yer gear ratio is to low,.... you can't buy a bigger prop,... yer rpms are too high, 'n speed to low,...

 

[Wave thumper]

Re: New Build complete - Performance results of first test

Ayuh,.... Yer gear ratio is to low,.... you can't buy a bigger prop,... yer rpms are too high, 'n speed to low,...

Ok, so where should my RPM be???

Max RPM is 5000 which I thought was spot on for 5.0.

 

[Bondo]

Re: New Build complete - Performance results of first test

Ok, so where should my RPM be???

Max RPM is 5000 which I thought was spot on for 5.0.

Ayuh,.... Generally speakin', the cam in boat motors, build power from idle to 4800 rpms,...

Yer just above that, 'n movin' out of peak torque,...

With a Higher gear ratio, aka; lower numbered, 'n a mid-range pitched prop, you can easily dial in the motor, 'n hull to Wot, at 4800 rpms,...

A V8, in that tiny hull, really needs the 1.50:1 gears to reach any kinda potential,...

If the hull is in any kinda shape, ya oughta be pullin' 50 mph,...
or ya mighta mounted it Too low,...

 

[Wave thumper]

Re: New Build complete - Performance results of first test

Ayuh,.... Generally speakin', the cam in boat motors, build power from idle to 4800 rpms,...

Yer just above that, 'n movin' out of peak torque,...

With a Higher gear ratio, aka; lower numbered, 'n a mid-range pitched prop, you can easily dial in the motor, 'n hull to Wot, at 4800 rpms,...

A V8, in that tiny hull, really needs the 1.50:1 gears to reach any kinda potential,...

If the hull is in any kinda shape, ya oughta be pullin' 50 mph,...
or ya mighta mounted it Too low,...

Cheers Bondo, your a wealth of information.