Is 29mph what I should expect?

[levelution]

I have a 1976 15' tri hull with a 1976 70 hp Johnson. The previous owner put in a new speedometer and per that at wide open throttle I can get to 29mph. Is that pretty good for this size engine in this boat? Or should it be faster?

 

[JasonB]

Re: Is 29mph what I should expect?

Have a friend that had a similar rig last year. The boat sat very low in the water compared to my tri-hull Glastron, even on plane. I could do 36mph with my 85 and would leave him. That being said, it really depends on the boat, but 29 isn't out of reasin as that was about what he could do. Make sure you don't have a bunch of extra weight like wet foam. A Hydrofoil or SmartTabs may help.

 

[Silvertip]

Re: Is 29mph what I should expect?

Without a tachometer, you don't know if the engine is reaching its proper wide open throttle rpm range (5000 - 5500). Pitot tube speedometers are notoriously inaccurate so don't go by that. Use a handlheld GPS or run along side a friend with a known accurate speedometer.

 

[ED21]

Re: Is 29mph what I should expect?

Way back when-that's exactly what our 16' tri hull<br />w/a 70 johnson would do. No GPS then(we had an 8 track on the boat if that gives you a clue)just a pitot speedo.<br />RPM was 5200 as I remember.

 

[sprouticus]

Re: Is 29mph what I should expect?

i have a 16' with a 1977 70 hp evinrude. It isn't a trihull, but I thought it was close enough. I also get about 29 MPH, could get her up to 30 with a good tail wind, lol. how is your boat plaining??you may be able to go with a higher pitch.

 

[sprouticus]

Re: Is 29mph what I should expect?

i have a 16' with a 1977 70 hp evinrude. It isn't a trihull, but I thought it was close enough. I also get about 29 MPH, could get her up to 30 with a good tail wind, lol. how is your boat plaining??you may be able to go with a higher pitch.

 

[Silvertip]

Re: Is 29mph what I should expect?

If the engine was turning 5250 at WOT, a slight decrease in prop pitch would put the engine in the 5500 - 5600 RPM range. The engine will be happier and hole-shot should be improved.

 

[CATransplant]

Re: Is 29mph what I should expect?

Hard to say whether that's as fast as that boat will go. I also have a 15' trihull (see below), but it's a light one (700 lb). I have a 35hp motor on mine. It does 25 mph (GPS) at full tilt with just me in the boat.<br /><br />It depends on hull design, weight, and about a zillion other factors. The only way to tell, really, is to have a tachometer on board so you can tell if your outboard is reaching its rated RPM at WOT.<br /><br />If it is, then you're probably hitting the right speed, even if it seems a little low. If not, then it's time to re-prop and get a few more MPH out of your boat.

 

[ledgefinder]

Re: Is 29mph what I should expect?

The current 15-footer I run weighs maybe half what my old Larson 15-footer weighed. The Larson had this thick foam sprayed all along the hull as floatation, & I suspect it had taken on water. That Larson needed a 115 to really kick it, while with the present boat a Merc 50hp is enough. [Needless to say, the 115 shredded the Larson's transom, but it was fun while it lasted...] The difference was also very evident when towing the boat.<br /><br />Make sure the 70 is propped right - should turn around 6000 with you alone in the boat, and check the compression. But otherwise, should be OK.

 

[levelution]

Re: Is 29mph what I should expect?

Wow, thanks for all the responses! I feel a little better about the speed now, but I think I am going to see if I can hook up a tach to it and check the RPM at WOT.

 

[REELSPORT]

Re: Is 29mph what I should expect?

I have a sixteen footer trihull with a 55hp rude on it and I get 31mph gps at 5300rpms

 

[Tatorbug]

Re: Is 29mph what I should expect?

I wouldn't be dicouraged with the way your boat is running. Seems pretty much average speed as compared with my friends similar setup.

 

[kirkland]

Re: Is 29mph what I should expect?

My 17' cajun trihull with 70hp and 13 1/4 X 17 aluminum @ 5800 rpms I get about 36 GPS with two adults and about 400 lbs of other stuff...

 

[sprouticus]

Re: Is 29mph what I should expect?

hmmmmm.....same prop as me, same motor, bigger boat, but faster..........think I will do a compression check.lol