Prop size on a Tomcat w/Honda 150s?

C-Roamer

Hello All,

I received my Tomcat on the ferry from Juneau and took it out for the first time last week. I only had 8 hrs on the motors so I didn't open it up for long but, it would only reach 32 knots at 3,800 rpm. That was WOT.

We were heavy, with 4 persons, full fuel and full water.

I have the S 3 15 1/4 x 17 props.

So does that sound like a prop issue? If not, what else might it be? Linkage?

Thanks,

Captains Cat · Posted: Sat May 08, 2010 1:00 pm Post subject:

David, I don't have Hondas on my TC but something is definitely amiss. It should be turning 5500-5800 or thereabouts. I know there are others here with those engines on a TC, I'm sure they'll chime in soon.

Charlie

thataway · Posted: Sat May 08, 2010 9:55 pm Post subject:

As Charlie, we don't have a Honda, and the Suzuki's have a much lower ratio in the lower unit, so they can take much higher pitch prop.

You do want to get the RPM in the 5500 range. You may be lugging the engine some with the higher pitch prop. Also check trim, to be sure that you are not bow down attitude too much. Trim up until the Tach goes not go up any more (but don't over trim so that the engine cavitates. It also may be that the engine is set too deep. You want the engine to run just at the surface of the water. With the engine bracket, you set the engine up at least one inch for each inch back from the transom. So for the Tom Cat you want the engine 2 to 3" above the bottom of the boat--you can check this on the trailer. --level the boat (use a level)--and then clamp a level to the cavitation plate, it should be a couple of inches above the bottom of the transom, as it butts up to the transom.

matt_unique · Posted: Sun May 09, 2010 5:57 am Post subject: Bolt holes

Hi Bob - along these lines - what bolt position are your engines set? If each hole from top to bottom was 1, 2, 3, etc. what are yours set at? Mine are set on hole number 1 - meaning they are as far into the water as possible. My RPM's are in the green zone when trimmed up - but I may have my engines lifted a hole or two one of these days if other owners have good results with a similar placement.

Thanks

C-Roamer · Posted: Sun May 09, 2010 1:46 pm Post subject:

My bolt holes are also on the No. 1 position or as far into the water as possible. I forgot to mention in my original question but I also have Permatrims installed.

thataway · Posted: Sun May 09, 2010 7:17 pm Post subject:

Matt, I am on the road, but my recollection is that they are either in first or second hole--(I actually took a photo, but not on this computer).

C-Roamer · Posted: Thu May 20, 2010 6:56 pm Post subject: Follow-up

We splashed the boat today and tried again after finding that the tack was not properly calibrated. The rpm came up to 6,000 - 6,200 at a speed of approximately 40 mph. This was with three persons, full fuel and full water. The prop that is installed is a SS 3 x 15 1/4 x 17.

I am satisfied with this and it is set up correctly for the amount of weight that I will likely have on board.

thataway · Posted: Fri May 21, 2010 7:07 pm Post subject:

You gave the speed in knots (32) the first time and RPM at 3800. The second time you gave speed in mph (40) and RPM as 6000 to 6200. 32 knots is about 36.8 miles per hour. Not a lot of difference, but significant. Any suggestion why the difference? Was the tach configured for a 4 cyl engine ?

Your top speed is a bit below what some of us get with the Suzuki's, but again it depends on weight in the boat. A boat with full fuel water and 4 big guys, vs perhaps a boat full of fluids, plus a lot of cruising gear--maybe another 500 to 700 lbs? My impression is that the Honda's run a little slower. What is your RPM at cruising speed of 25 mph?

C-Roamer · Posted: Sun May 23, 2010 11:48 am Post subject:

Hello Bob,

We took the boat out yesterday. According to the dealer, the tack was not hooked up correctly. Our rmp came up to 6,300 or so. I am not sure of the specifics.

At 3,500 we were getting approximately 20 knots. We were heavy and there was 2 - 3 foot chop.

Thanks for your comments.

thataway · Posted: Sun May 23, 2010 6:25 pm Post subject:

The 3500 and 20 knots sounds about right--I think that you will do well with that propping.

Have fun.

Dreamer · Posted: Sun May 23, 2010 10:58 pm Post subject:

Dreamer gets about the same speed for that RPM. We have 15x19 Stainless props on the 150s. Do you know what your fuel burn was at that speed? We normally cruise at 22 kts and burn 10.5-11.0 GPH at that speed.

C-Roamer · Posted: Tue May 25, 2010 1:22 am Post subject:

Roger, no I don't know my fuel burn yet. I will get it dialed in and let you know later in the season.