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  MONTAUK: Tohatsu 90 TLDI

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Author Topic:   MONTAUK: Tohatsu 90 TLDI
Sonic posted 06-20-2006 11:09 AM ET (US)   Profile for Sonic   Send Email to Sonic  
The Montauk [was] in the water last weekend, and I decided to get some accurate performance numbers for it, as I am looking at getting a new prop to replace the one that came with the motor.

Boat Specs:

--1983 Montauk
--2005 Tohatsu 90 TLDI
--full 24-gallon fuel tank
--one person (200#)
--limited gear (Anchor, battery, not much else)
--bottom is painted, but clean and without buildup
--propeller is the original Tohatsu 13.25x17" aluminum one
--motor is mounted as low as it will go in the blind holes
--motor has a 2.0:1 gear ratio and a max RPM range of 5150-5850
--motor weight is 315-lbs
--I was sitting for all tests and out of the wind. When I stood up, there was a 1mph difference in speed at the top end.
--water conditions: Waquoit Bay, MA, 10AM 6/15. SW wind ~5mph, ebb tide near low, surface ripples on water, runs made both ways for accuracy and averaged.
--trim for all was "neutral" where the motor would track straight with no input, and trimmed up for the fastest 2, or it would not go beyond 4800 RPM.

RPMx1000 Speed (MPH)
1 4.9
1.5 6.6
2 10.2
2.5 16.2 on plane
3 22
3.5 27.6
4 31.7
4.5 36.1
5 38.8 trimmed up
5.2 40.4 trimmed up

The boat is used for running around Nantucket sound and [unrecognized acronym] Sound, going to the beach, and water skiing and tubing. I think a 15-inch pitch SS prop with a 5-inch setback jackplate to get the motor up a bit and out of the blind holes will get me much closer to the upper end of the [recommended wide open throttle engine speed] range, and better [acceleration].

--Chris

ratherwhalering posted 06-20-2006 01:03 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Your performance numbers look good for a blind hole mount. If you add set back brackets/jackplate and raise the engine, you can expect a 300-400+ RPM increase, depending on how high you raise the brackets/plate. That being said, theoretically, a stainless prop will increase your WOT RPM, and slightly decrease holeshot. Stainless props are much heavier and stronger than aluminum, therefore, it will not spin as fast when accelerating, but will not flex as much at higher speeds. However I have noticed that stainless props (in general) tend to have greater cupping and less slip than conventional aluminum props, so you may not notice any decrease in hole shot, but you will see an increase in top speed.

There is a rule of thumb which holds that every two-inch increase in propeller pitch will decrease engine speed by 450-RPM, and vice versa.

Therefore, I think you should stick with the same size propeller, but in Stainless, or you may over-rev the engine. 5600 RPM out of a maximum RPM of 5850 is pretty darn close. If you really wanted to get to the maximum RPM, then perhaps try CMC 4-inch set-back brackets, with a Stainless Tohatsu 13.25 x 16 (NM889610160.) Otherwise you may want to look into:

Tohatsu 13.25 x 17 (NM889610170) for about $350.00
Michigan Rapture 13.25 x 17 for about $300.00
Stiletto 13.25 x 15 for about $240.00. (acts like a 17-pitch)

Perhaps someone more familiar with Mercury propellers can suggest additional options from that manufacturer?

Teak Oil posted 06-20-2006 04:18 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
Be sure to get some mileage numbers once you get dialed in. I dont think there are any other Tohatsu TLDI's on this site
bsmotril posted 06-20-2006 05:07 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
Thanks for posting. I've been considering a Tohatsu or Nissan 90hp and this is the first good performance report I've seen for that motor on a Whaler hull. BillS
Sonic posted 06-21-2006 11:26 AM ET (US)     Profile for Sonic  Send Email to Sonic     
Thanks for the comments.

I don't have any way to measure fuel economy, but I know one thing: it is FANTASTIC! It seems to burn close to 1/2 what the old Johnson 90 would burn. Going water skiing for a day used to burn up most of a tank of gas...now it uses less than 1/2, and at a 3500 rpm cruise it just sips gas like we're running out. I would estimate oil use at about 60:1 so far.

Overall, I've been very happy with the motor, less happy with the quality of work from my dealer. I think a setback/jack plate is in my future.

goldstem posted 06-21-2006 01:18 PM ET (US)     Profile for goldstem  Send Email to goldstem     
Waquoit bay. I grew up there. in a house next to Kelly's clam shack on the seapit river.

perhaps we should do a cape outing later this year?

m.

Teak Oil posted 06-21-2006 04:50 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
If you have a GPS you can measure avg mpg. Next time you go on a 25 mile plus trip fill the tank and fill it again at the end. Whammo instsnt mpg
The Judge posted 06-22-2006 12:17 PM ET (US)     Profile for The Judge  Send Email to The Judge     
Old 90's burned about 6gph @ 4k. Yours is probably burning about 3-4. Easiest way for me to measure is using a 3 gallon tank and seeing how long she goes before running out.
jimh posted 06-22-2006 08:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the information on the TOHATSU motor. For the TLDI models TOHATSU have licensed the use of the same Orbital Combustion Process that Mercury licenses for use in their OptiMax models. Tohatsu is also the primary manufacturer of most of the smaller horsepower four-stroke motors sold by Mercury. They make the motors which are re-badged as Mercury motors in North America. Mercury and Tohatsu are also involved in some joint manufacturing operations in a new plant in Japan.

I do note that Tohatsu seems to be promoting its own brand in North America in a more prominent way lately. There have been prominent Tohatsu brand advertisements in boating magazines which are oriented to smaller, trailerable boats.

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