cruising speed query

[wayne]

Have a 2000 tomcat with twin honda 100's. I notice at the factory website that this boat with twin 90 is expected to max out at 38 mpg. Mine has nothing near this speed. Running in flat seas at 4800rpm, and tweaking the trim I am able to run at about 23-25 knots. Is this normal, and if not is there a suggestion as to what might be the problem.

Many Thanks,


Wayne

[C-Bill]

[lyle-t]

Not to be mean but MPG ( MPH maybe??) and KNOTS do not compare. But then who said I was smart anyway? Just having some fun today so please do not get mad at me.


Lyle

[rogerbum]

The factory site claims - 37.0 MPH, 2.26MPG and 31.8 knots with twin honda 90's and 1200 of fuel people and gear using 13 3/4" X 17" Propellers (3 blade, Aluminum). Your speed does seem a bit low - what props are you using on your 100's? and what it the typical load on the boat?

[Forty Two]

Where did you find Honda 100's and how did you get them to turn so slow?

My TC with Honda 90s maxes out at 28 to 30 knots, 5800 rpm. That is 32 to 35 mph for you landlubbers.

[gljjr]

Just going by the RPM numbers you state I would say you have the motors propped wrong. Either that or they are set too low on the transom or both. Those engines need to get up to at least 5200 rpms and closer to 5800 would be better from everything I have read.

[katkt]

Mine runs similar to Fishbobs. I've got the factory bottom paint. I'm a landlubber with a landlubber GPS, and I've seen 37 mph. It could have been caused by a tail wind and that lighter than air feeling you get whilst motoring about in your new Tomcat.
Check on the props for the diameter and pitch(should be stamped on there somewhere similar to the way Rogerbum has it listed 13.75 x 17 etc.), and see if you can find something regarding the WOT(rpm) for those motors.
By reducing the diameter and or pitch you will get an increase in rpms. Find out what you have to start with

There was a couple of 100's listed for sale on several hits after I googled it. Huh! Who'd a thunk.

[wayne]

Okay, my mistake and my typo. I didn't mean MPG, but rather MPH. I also, being new to the boat, stupidly put the wrong engine type. I'll start over. The engines are twin Yahama 100's. I was told, maybe by those who don't know, that they should run at about 4800 rpm. At this rate I get about 23 - 25 SPG, or speed over ground measured by the GPS. I was simply wondering if this was normal.

Wayne

[gljjr]

Wayne,

Looking at the specs for the 2000 Yamaha F100 they are rated to run between 5000 and 6000 rpms (Page 58/59 of the owners manual online). I was able to view the owners manual by going here... http://www.yamaha-motor.com/outboard/service/viewmanuals/viewmanuals.aspx?ls=outboard

Given that you are running at top rpm of 4800 it would seem that you are lugging the motors down and not getting optimum performance from them. I would seriously consider changing props to get the rpms up to the middle of the specified range in the owners manual. That should give you better fuel mileage and more likely better top end. But do make sure that the cavitation plate is in alignment with the bottom of the transom or up to 1" above the bottom of the transom first. If it is lower than that you will lose a lot of performance due to excessive drag.

I've read a lot of BBS posts on tuning propellers to the application and I am by no means very knowledgable on this (Hey I have a Jet Sled right now!). Having said that I would contact one of the propeller manufacurers to verify what I have stated before doing anything. But to me it sounds like you should have much better performance from those motors.

[katkt]

Wayne,
Your motors should be running no less than 5500-5800 rpms at WOT(wide open throttle)when the boat is lighty loaded(fuel and 2 people)and the water is calm.
Did you check the props to see if you could find the diameter and pitch?