Contradiction in prop information - Tohatsu 25 hp

[tschwer01]

I have a 2004 25 hp, 4 cycle Tohatsu powering a 14' alum runabout. It currently has a 9.9 x 11 steel prop. The problem I have is that I can only get the boat up to 15 mph (per my Garmin) carrying 2 men and a cooler (plus 6 gallon gas tank). The boat never gets on plane. All I want is to get on plane and be able to go over 20 mph at WOT. I like to travel from Augusta, ME to the ocean and my usual 12 mph, no plane is a huge annoyance. I am asking for advice on what I can do, if anything. It seems like the standard rule is that i higher pitch will get me greater speeds at WOT. I do not need fast starts, we are just fishing for stripers and blues in the Kennebec.

Tohatsu has a chart for my motors prop that says you can go to a 14" pitch for light loads. The heavy loads are recommended to have a lower pitch. Tohatsu Outboards: Authorized North American Distributor for Tohatsu Outboards, Parts, & Accessories.

Do I have a light load? Can i buy a different manufacturers prop? I see they offer a 10x15" with 4 blades. Will this get me on plane? 4 vs 3 blade?

I did have a mechanic verify the compression is perfect.

Thanks a lot. This has been a great source of frustration for me. :grumpy:

Tim

 

[steelespike]

Re: Contradiction in prop information - Tohatsu 25 hp

A suspect something other than a prop.
The first thing I notice is The max moderate weight is 1500 and the minimum heavy weight is 1500.
With a 10" ok for one and a 9.3" for the other. light 100- 500, max of 500 with a 13' prop and the minimum moderate at 500 is a 12" So there is an overlap.If the 11 turns out too much prop you need to reduce pitch to a lower number but I still think that its not a prop issue?
You have to work with the total weight to figure the prop.
We need boat,motor passengers and gear total don't forget anchor,oars,battery?, trolling motor?etc.
A runabout usually has a floor,deck,windshield,remote steering maybe even a live well.
assuming your boat is a bare utility your total weight could approach 1000 if you have a lot of gear.
That said a 25 should manage that load and I think the 11 inch is probably in the range what you need.
If it is a ss prop an 11" might act like a 12" because of the cup.
A 4 stroke needs to be propped near the max rated rpm If you allways run with a good load prop it for that and don't over rev when light.
What does the boat do lightly loaded?Rpm and speed is important Also need your gear ratio.
Some easy things to check. Have you tried different trim settings?
Is the motor leg length the right size? Antivent plate(just above the prop about even with the bottom)?
Is the throttle opening all the way?
Is the boat a full sized 14 with 14+ feet centerline length?
Smaller boats sometimes struggle getting up with a heavy load.

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

Usually it's just me and a dog. My profile picture shows the boat (steering, battery, no live well, not much else) Nothing special, 14' aluminum. It runs fine in a low gear, stays flat. But as soon as I move the throttle forward the bow kisses the sky and will not come back down unless I have the wind at my back and can get it over 20 mph.

Can I do these RPM tests in my driveway? I do not have a tachometer, but can buy something. The current prop is pretty beat up from unmarked rocks so I need a new prop anyway. Just trying to figure out if going from an 11 to a 15 pitch is what I want to do. I'm going to remove the nice wood deck I put in and save 50lbs, just have the marine board only.

thanks for the help,

 

[Slip Away]

Re: Contradiction in prop information - Tohatsu 25 hp

Your diameter is too small. Should have a 10-1/2 x 10 aluminum 3 blade as stock for 14' boat. Even SS prop should be 10-1/8 x 13 3 blade.

 

[steelespike]

Re: Contradiction in prop information - Tohatsu 25 hp

Near as I can tell your diameter is fine an 11" would likely have a smaller diameter than a 10" pitch.
Have you tried trimming down?(prop closer to the boat)
Getting the rpm and speed has to be done on the water.
A 10" prop will certainly help your problem.But it may be a bandage over an existing problem.
You could try the 10" the worse that would happen is you end up with a spare prop.

 

[Silvertip]

Re: Contradiction in prop information - Tohatsu 25 hp

You CANNOT do wide open throttle tests in your driveway. Think about it. The engine with no prop will rev to the moon just as it will in your driveway with whatever prop you put on it. LOAD is what the engine needs. That means it must be in the water with what YOU consider an "average" load. You then make several wide open throttle runs noting speed (by GPS) and engine rpm. Without RPM numbers you have no idea what the engine is doing. The reason that's important is that if the manufacturer recommends a WOT rpm range of 5500 - 6000, and you can only achieve 5400. You need LESS prop pitch -- not more. Conversely, if the engine revs over 6000 you need more pitch to bring rpm down. Since your bow pitches up at high throttle settings, you need to pay attention to trim. You may have the trim pin (manual trim adjustment) set wrong, or are using too much out trim for hole shot. You may also have a long shaft motor on a short transom boat. If the leg is correct for the boat, then check the position of the AV plate in relation to the bottom of the boat. It should be even to 1-inch above the keel. Checking this stuff is all free. Don't buy props until you get the setup correct and you know what rpm the engine is currently turning.

 

[Tail_Gunner]

Re: Contradiction in prop information - Tohatsu 25 hp

{It runs fine in a low gear, stays flat. But as soon as I move the throttle forward the bow kisses the sky and will not come back down unless I have the wind at my back and can get it over 20 mph. }

Could you take a picture of the transom with the engine in the down or in the running position.It sound's or reads like your leg or drive is running a bit deep. If the engine is to low it will Suck down the boat...just as your describing.

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

Thanks everyone for the suggestions and advice. It sounds like I need to do some verification of existing components before I buy a new prop. I do not think the snow is covering my boat yet, so I'll take some pictures tonight after work. Thanks.

BTW, I tried the trim in all 6 different settings and it never made a difference. I was told by the marine shop that I need a long shaft boat. That's about all they were concerned about. I wouldn't be surprised if I am running too deep with all the rocks I've hit.

Any recommended tachometers?

thanks.

 

[steelespike]

Re: Contradiction in prop information - Tohatsu 25 hp

Induction tachs liker the Tinytach are inexpensive,easy to install and will work on almost any motor.

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp



Here are the pictures of the outboard at the highest and lowest setting. does this look correct?

 

[Tail_Gunner]

Re: Contradiction in prop information - Tohatsu 25 hp

I expected more of a drop but pic's can be deceiving..it appear's 2 " low...Lets see how the other's respond Could be 4 ' to low

 

[steelespike]

Re: Contradiction in prop information - Tohatsu 25 hp

Looks a couple inches low to me.Lay as straight edge along side the keel out to the anti vent plate.
Get down to the antivent plate with the camera. Generally a 20" transom isn't exactly 20 inches
and a 20" motor isn't 20 inches The idea is you can fine tune it to the right height for your application.
I would suggest raising it till venting becomes a problem.
If you don't experience any venting the motor probably is too low.

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

Here are some more pictures in case it helps. i appreciate this.

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

ok, i'll try the straight edge tomorrow. I believe the boat naturally sinks toward the stern due to the 4-cycle motor compared the the 1960s 2-cycle it was designed to use? Did it look more correct in the high trim position vs the low position?

 

[jestor68]

Re: Contradiction in prop information - Tohatsu 25 hp

The motor height is not was is keeping your boat from getting on plane; you have too much pitch in the prop.

You need to consider that boat to be a heavy load for your 25 hp motor. In fact, the motor is telling you that right now because it cannot muster enough to spin that prop up and get you going.

Set your trim pin in the first or second hole( in toward the transom).

Drop down to a 9.8 X 9.3 and you'll get going. A tach will be needed to verify that you don't go past 6000 rpm(which I don't think will be a problem).

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

I think you just answered the whole "contradiction in prop information" for me. I read that more pitch = more speed at WOT, but technically I have a heavy load so that will never work. I always thought I had a light load since it's an aluminum boat, but factor in the 25 hp and now I can see where I went wrong.

I'm still going to post some pics later with a straight edge to verify my motor isn't too deep, but I think we are onto something here...

Thanks for answering.

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

Winter storm in town so my flash-less camera only takes blurry pictures during overcast. I laid my level flush against the underside of the AV Plate. Looks like it comes within 1-2 millimeters of the bottom of the keel. I'm in tilt position 2 from the stern. If I kept the instrument level then the rear of the AV Plate would be an inch or two below the level. Any advice? Thanks.

 

[Tail_Gunner]

Re: Contradiction in prop information - Tohatsu 25 hp

Raise it two inches and note the result's...the upside..at least it's not a 400lb engine..Even i/o's have the cav plate 1-2 inches above the keel.. The ideal is to raise the engine until it becomes non functional...aka the prop blows out on turn's easily..some keep it there and just slow down to turn

 

[Tail_Gunner]

Re: Contradiction in prop information - Tohatsu 25 hp

There's a shot of a I/O drive and they run very low in the water...The cav plate is 1.5" above the keel and it looks like your cav plate is 1.5" below the keel

 

[tschwer01]

Re: Contradiction in prop information - Tohatsu 25 hp

I'm sold. As soon as the ice melts I am raising the motor and see how she runs. I think raising the motor and lowering the pitch are going to get me on plane. One step at a time, but I am strongly encouraged by all the feedback. Thank you all so much.

Here is my slow gal on the Kennebec
youtube.com/watch?v=D1eAXdqBHCw