Not getting a desired speed

[hunter24]

So I bought a boat a few weeks ago, its a 17ft Grumman hull with a 1996 tohatsu m40c motor. I ran a speedometer on my phone, and with me and another person, I was only getting 9-11 mph and 17 with just me. Is there a way to get more speed out of it? I've been looking around and people have been saying they get around 30-40mph.

 

[alldodge]

More speed depends on several things

Health of the motor - is the motor producing good power or is it tired
Reaching WOT RPM - is the motor reaching manufactures wide open throttle rpm
Boat weight - Is the hull dry inside or are the transom and stringers water soaked
Is the hull clean
Is the ventilation plate even with the bottom of the keel

What is the max hp plate list on the boat?

 

[Sea Rider]

Welcome to IBoats,

Will begin saying that without an induction tach istallalation won't be seeing any light at the end of the dark tunel any time soon. That motor revs 5200-5800 wot rpm factory stated, to achieve just 17 MPH while lightly loaded must be running towards the extreme lugging side of the wot rpm range. Need to know if motor revs at least middle to max wot rpm range

To know the basics :

Is this a tiller clamp or remote clamp motor installation ?
Max stated HP and max number of passengers ?
Was the motor revving high when at just 17 MPH ?
What's the current pitch number stamped on prop body or blade ?
Is the motor producing strong sparks on both plugs, plugs are old, new and correct ones gapped to 1.0 mm tight?
Does the time advance plate mark stops at 25 deg when at full throttle ?
How about compression ?

Tohatsu motors works with different height parameters compared to others brands in order to work spot on, achieve their max HP cowl stated along max prop thrust on any water cond from flat to windy choppy ones.

Happy Boating

 

[WesNewell]

Assuming this is the boat, it's rated for a 70 hp engine. So you are under powered by almost half.
http://boatspecs.iboats.com/Grumman_by_OMC__G1749B_Outlaw_Bass__1996/bp/66b40588
And this is your engine.
https://www.tohatsu.com/tech_info/own_man_pdfs/Toh40C.pdf
You probably want a 13-16 pitch prop depending on load, heavy to light. And should see 30-37 mph if all is in good condition.

 

[Sea Rider]

Hey WN,

Why is it that you always surprise me and assume others as well, the OP has not answered any of the points asked previously and you want him to go straight to change props. Those illogical answers are not the way to go for, not even consider. That's why some posts doesn't progress fast due to this type of nonsense answer which I imagine is the product of extreme confinement and boredom.

Any underpowered (if being so) Tohatsu can be maxed propped to run at their max wot rpm range, don't pretend to go boating with the max number of boaters and max motor rated for if the motor happens to run a higher HP motor rated for. Won't have a speed machine but will have tons of fun once that motor runs strong again..

Happy Boating

 

[QBhoy]

Hi
You sound like you are not planing with a few onboard and are with just you.
I’d be almost certain that even with the perfect prop, you won’t see anything like 40 mph.
I agree you sound like you are both underpowered and over propped...
as sea rider has said...you really need to know what rpm you are getting and what pitch you have now.

 

[flyingscott]

You need a bigger motor a 40 hp is just not going to do what you want. Post 4 is wishful thinking.

 

[WesNewell]

SR,
Nowhere did I suggest he change props. Why would I when he didn't even say what prop he had. I just provided info based on what little info he provided. If you don't agree with it then provide something different.

 

[Faztbullet]

It will run 40+ behind a good truck!!! But in the water you will be lucky to get 20MPH with that set up.

 

[ahicks]

I have a couple of 17' alum. boats (one in Florida, and one in Michigan) I use for testing 40-50 hp Honda's that have had serious repair work done on them. Both are side console fishing boats with casting platforms front and rear. One is a semi V hull, the other is one of the popular "jon boat" types. Neither are over weight with water logged foam. Anyway, both boats will run high 20's and low 30's using 13-15p props.

Point being, I think it's pretty safe to say that a generic boat this size, running a generic 40-50hp motor, turning a generic 13-15p prop, could be expected to run in the mid 20's to low 30's MOST of the time. Sick motors and water logged foam would be the exception. You just don't need a lot more info.

That said, if you want to dial one in for max performance to satisfy a particular mission you have in mind, a tach is necessary. No if's and's or but's to it.

 

[WesNewell]

If he has the boat I listed it only weighs 675# and if the hull and engine are in good condition, he can get at least 30 mph out of it by himself with a 13P prop at sea level. If not, then something is wrong.

 

[flyingscott]

If he has the boat I listed it only weighs 675# and if the hull and engine are in good condition, he can get at least 30 mph out of it by himself with a 13P prop at sea level. If not, then something is wrong.

How does that package only weigh 675#s.

 

[ahicks]

Common sense would have just the hull at 675....

 

[jimmbo]

If he has the boat I listed it only weighs 675# and if the hull and engine are in good condition, he can get at least 30 mph out of it by himself with a 13P prop at sea level. If not, then something is wrong.

30 mph on a 17ft with only 40hp? You gotta be kidding

 

[WesNewell]

Dry weight of boat is only 675#. Add 300# for person fuel and gear. 129.95# for engine. 30 mph is easy. 35+ mph is doable.

 

[WesNewell]

[h=3]Your setup[/h]

• Boat type: Bass
• Usage: Overall good performance/0 ft
• Engine Manufacturer:
  Mercury
• Engine Type: Outboard
• Engine Year: 2017
• Number of Engines: One
• Engine Family: FourStroke
• Engine: 40 EFI 3cyl (747cc)
• Drive / Gear Case: 30-40 HP 2cyl 2-S, 30-40 3cyl 4-S, 2.0 Ratio

[h=3]Details[/h]

• Calculated Pitch: 14.82
• Calculated Weight with engine(s): 1,225.20 lbs
• Calculated Engine RPM: 5,750
• Calculated Boat Speed: 36.43 mph

 

[Sea Rider]

Let's keep flying over the cuckoo's nest with all kind of answers, the OP has not answered a thing about what's beer tech requested as a starter as there's no magician available to fix that motor any time soon. Before going any further motor must be running strong again,once there, the combo needs a spot on motor/transom optimization and soon after a prop maximization to get the max HP out of it.

Hey WN, thanks for posting the Tohatsu Ownwer's Manual, Page 2, point 2 (Transom Height) states that the AV plate must be set 1.5 to 2.0 inches lower than the middle rear hull, so why in God's sake does eveyone likes posting that must be even with rear middle hull. Will remaind you all that all Tohatsu motors have way taller lower leg plates compared to other brands. So cut the cast on stone crap that must be even with rear middle hull...

A dry installation will show nor prove nothing, once the boat is on plane it's a different scenario. This is the ideal motor/transom height setting for Tohatsu motors powering anything that floats with a motor, works spot on all water cond. If in need to raise or lower the motor on transom to duplicate that precise setting it's a different story..

http://www.youtube.com/watch?v=oBeZj...ature=youtu.be

As I can tell, not assume, none of you guys have in use or know well these 2 strokes Tohatsu motors. A 2 stroke 30 HP running strong, spot height seated and max propped runs as much as 30 MPH on a medium light boat, that 40 should run way more, the issue is that isn't known so far what's the max HP and number of passengers rated for that boat model.

This BW 15 footer boat powered with same 40 HP motor and stock prop with just a optimized motor height is super fast and darn scary running at wot and probably not revving to its max wot rpm range which isn't known.



BTW, the lower leg when the boat is on plane matches what the YouTube shows. If you see the AV plate being even with the rear middle hull, sorry you need prescription eye glasses ASAP.

My 2 stroke 18 HP powering a half ton 450 Mtr boat achieves 21 Knots at max at 5800 rpm range with 2 up and a 40 HP has double CC powerhead compared to a 18 HP. As long the OP doesn't report what's been previously asked, this post will fly higher than what a kite can fly...

Happy Boating

 

[hunter24]

The hull is rated up to a 75hp, but I cant afford one right now. The motor seems to be running at ~60-80% rpm to what I've heard it while revving at neutral (sorry, dont know how to explain it without a tachometer and being new to this). And I am looking into a new prop due to hitting a log and bending it. What would be the best prop for this motor? And the plate above the prop is level (or around it) to the bottom of the hull. I've adjusted the trim to multiple angles and the one I've got the fastest speed I topped out at was 17-18mph. I can try to take some pictures at some point if that will help. Thank you for all the help.

 

[flyingscott]

Revving your motor in neutral means Nothing!! First thing is get a tach and replace the prop with the same one you have. Once you have a tach and a good prop you can go from there. Your combo will never be the speed demon you want it to be.

 

[WesNewell]

Somewhere on your current prop, there's a number. What is it? Do a search for it. Without knowing what you have now and without a tach It's hard to say what you need. Could be anywhere from a 12p to a 15p. IF you just want to get going, I'd suggest a 13P. That should at least get you on plane. You really need a good prop and a tach to see what you need for max performance.