Props for Tom Cat

To answer the "backwards turning" prop question , We have found with planing type catamarans that there is a slight gain in speed [probably related to "bite" in cleaner water,or lack of slip] and no steering downside as you would have with closely spaced props on a monohull . Not exactly sure why , but we have done it this way for years on our Tomcats and Twin Vees and Sea Cats , and it works ! Dont know if it would make a difference on a pure displacement Cat like a G.B.
5700 is quite acceptable [ for a 4 stroke Yamaha] RPM with a load in the boat . The pitch Vs. Diameter you have is good . What you can gain by changing blade shapes is the ability to trim the bow higher . That is important on Cats . I usually suggest folks to trim up [ at speed] till the motors cavitate , and then back it down till they bite again and note those positions if you have trim gauges or "time it" if you dont . You need to drive by feel . Dont be afraid of the trim buttons ,they are your friend.............
Marc
 
Wefings wrote:
Port motor on Stbd side and Stbd motor on Port Side.
Marc

Just be sure you know how your boat is equipt . I launched at Punta Gorda, after putting on the props. I had removed them to trailer cross country- too easy to have stolen. I put the boat in reverse to back off the trailer, and nothing but frothing the water. After a couple of trips up and down the ramp to look at the props. I finally realized that the right turning motor was on the port side and the counter turning motor was on the stbd. side. I removed the props and reversed the placement, and all was well again. I had always removed the props to trailer on our TC 24- mostly in Mexico- and that boat had the CC motor on the port side.
 
Brent , you oughta try it with 2 righthanded props , I had the pleasure of trying to get one off the trailer that way once . Its pretty twisty and torquey...........
Marc
 
Today the water was calm and I decided to experiment with trim a bit more. I brought the engines up until cavitation then back a pinch with WOT. I was able to get 6000rpm on the port engine and 5700 on the starboard engine, max speed 40.5 knots or 46.64 mph on the GPS. My boat lists to starboard right now (I assume from water tank) and I suspect if I trimmed the starboard engine up a bit more to compensate I would get a little more rpm.

I'm curious what others are reading for an exact max speed on the GPS? Bob I don't know how you are "well over" 40 knots! Your engines must be supercharged!

It was warm and humid today so perhaps in crisp fall air I might get a bit more. The top end is not particularly important to me but I want to make sure I'm in the right ballpark with other Tomcat owners. I'm very pleased to be running just under 47mph on a 28' (stem to stern) boat!
 
You are indeed at Fenway as far as the RPMs go.
Your transducer could be affecting the RPMs between the motors as well if its close to the middle of the sponson .
Marc
 
My last run, trimmed but not to perfection, through a very slight chop, was 39 mph at 5700 rpm WOT. Wonder if salt water is slower than fresh?

Warren
 
My transducer is an in-hull (shoot thru hull). Also my boat only sees salt water unless she is being rinsed!

I have bottom paint of course. I'm trying to remember now if Bob has bottom paint since Thataway is kept on a lift? If no, that would definitely explain a few knots at top end.

Warren, what are the specs on the Yam 150's? You're running those with peratrims if I interpret your pictures correctly. What are your props? I know the Suzuki 150 has the largest engine block for all 4-stroke 150's right now. I'm sure as with all technology this will change in 10 minutes but I wonder if that difference is enough to cause a 7 knot difference between our vessels? It could be several things though....props, amount and possibly type of bottom paint (my paint is hard, perhaps ablative paint would be slower through the water), gear aboard, etc.

As an aside, I learned what just a little slime can do to a boat. My former boat developed a bit of slime, not much at all, and my rpm was down about 1000rpm and the top speed to match. I thought for sure it had to be an engine problem. I had the engine checked out...compression...the whole 9 yards. No problems were found and nothing was done. I power washed the vessel and my rpm's and top speed returned. It was a significant change in performance, way more than you would think with just a little slime on the hull. I'm sure sail boat racers already knew what I had to learn but it was an eye opener.
 
On sea trials on our month old TomCat, the Evinrude Etec 150s had 3X15 1/2X15 counterotating props. We hit 46 mph with a relatively light boat. After filling the tanks, installing air conditioning, and all the other stuff for cruising, the max speed dropped of to 38 mph with the max rpm at 6200. Friday, the dealer installed 3X15 1/2X17 inch props. Now the top speed is 43 mph at 5500 rpm. This is for a relatively heavy boat in a two foot chop. These engines max out on torque at 5300 rpm so this must be the right prop for this boat at this time.
 
We did one with Suzuki 175s , and the owner tweaked the heights and brackets and is a 50 MPH plus Tomcat .He is a performance type of guy . Matt,the paint is probably the difference for you . In terms of top speed , Honda will be the slowest ,surpassed [and passed] by Yamaha and E tec , and Suzuki will be the fastest . All good motors , different characteristics .
Marc
 
Wefings":2ira31bq said:
In terms of top speed, Honda will be the slowest, surpassed [and passed] by Yamaha and E tec, and Suzuki will be the fastest. All good motors, different characteristics.
Click to expand...

Interesting. Can you tell us more about your opinion of the different characteristics of each in the 150 hp model?

Thanks,
Warren
 
Thataway is lift kept, and no bottom paint. At the upper range, this will definately make a couple of knots difference. Also engine position and weight.

I can only speak to the Suzuki vs Honda. The Suzuki 150 and 175 are built on the same block. The Honda 135 and 150 are one the same block. I don't have the displacement at my finger tips, but I believe that the Suzuki is slightly more. Also the prop is larger and with different gearing, has a greater pitch--all of these factors will make the Suzuki faster than the Honda..but only by a few knots, and there maybe a fuel use penality for the speed.
 
Sorry Warren , I missed that one . The Suzuki had the biggest displacement [513 CCs bigger than the Honda]and the high torque gearing [ big prop,more pitch,spinning slower]. Thats what makes it the fastest . So far for us ,trouble free. The Yamahas are 2670 CCs and much punchier than the Honda from idle and also trouble free. The Hondas are probably the most fuel efficient and the only one of the three that has variable valve timing. It comes alive in the mid 4000s when the timing changes but are noticeably weaker in the low end . That matters to some folks ,but not others .Its certainly not critical on a C Dory.They are also trouble free.I fish on a boat with a 150 Honda and it continually amazes me how little fuel it burns. We work on them all,sell them all ,and like the ability to offer the choice.I would happily own any of them . I might be a little partial to Suzuki though.......
Marc
 
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