Boat motor seems to make a huge wake . . . Tohatsu 20hp

[catfighter]

Hello - I have a 20hp Tohatsu short shaft on my 14' boat.

When I run the motor at 1/3 throttle, the propeller stays underwater but there is a huge amount of displaced water by the propeller. I'm wondering if this is bad; it seems to run fine and the telltail water is spraying. It just looks odd to me.

here's a picture of the motor on the boat; maybe it needs trimmed more or something??

https://sphotos.xx.fbcdn.net/hphotos-prn1/522135_10150985972816730_146328413_n.jpg


Thanks for the help!

 

[BonairII]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Your motor looks way too low. The cavitation plate should ride on the surface of the water at WOT (probably even or an inch above the keel line)

 

[Sea Rider]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

With boat floating on water, trim engine so to have Tohatsu sticker paralell to sea level, will need to determine where is water flow pasing with respect to tail when at full plane, if leg is too deep will produce unwanted issues at back engine. Small upper plate should ride slightly over water flow, not AC plate, at choppy seas will have excesive aireation, cavitation and even overheating issues as both water intskes will be too high near water surface.

Happy Boating

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

The boat was designed for a short shaft and this is a short shaft outboard.

I'll try to trim it more and see if that takes care of the issue. One would think that if the shaft were too long that the a/c plate would be running too low in the water, not too high.

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

So this might be helpful . . . Here's a video of my motor running:


http://www.youtube.com/watch?v=PU7_Smg4JE4


Does this look okay to you guys??

 

[Expidia]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

You asked a question. several posters gave you answers and then you came back with "it's a short shaft and this boat is made for a short shaft"

Which way do you want to go with this . . . your response or theirs. Your dragging the large cavitaion plate through the water! I had to go to a local metal shop and had them make up a block about 1.5 inches high and maybe 10 inches wide. This raised her cavitation plate (the large one) about even with the bottom of her 14 foot Monarch aluminum.

No more spray and handles like a dream now. The previous owners drove her rig for 10 years making a wake like your does.
Even looking down at the plate in your movie you can't see it as it's under water.

I bought a new boat in 2007 and I had to raise the motor up two holes in order to get the plate correct when in a vertical position and holding a straight edge along the bottom of the boat. Some dealers don"t care how they place and bolt down a new O/B or the unqualified people they have doing it don't know any better. My dealer was a Merc dealer too.

My Daughter's boat did not have holes hence the block I made for it for around $10.

Here is a link to pics of the bar I made and of it installed on top of her transom. I think I just used a screw on each end screwed down into the metal top to keep it from moving side to side. The weight of the OB holds it in place anyway.

Your motor may have extra holes to raise it up with.
Click on a pic to view it larger in photobucket:
The last few pics in the link below, show the finished product with the cavitation plate now being even or no more than an inch higher than the bottom of her boat. Too high and the prop catches air does not dig into the water around turns.

Show us some side pics of how your motor is bolted or clamped onto your transom and if there are any holes in the bracket to raise it and we can tell you the best way to raise it.

http://s198.photobucket.com/albums/aa2/expidia/1999 Monark/

The lower round circle is where the OB had been mounted for 10 years. As you can see, the metal bar raised it about 1.5 inches.

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Well, I didn't mean to "come back" with anything. I just assumed that you buy a 15" shaft outboard it's going to work with your boat with the 15" transom. . .

Luckily I have no bolt holes in my transom; I'll try to get a 1.5" aluminum block made and probably end up bolting it down to the transom instead of just using the clamps.

Thanks for the help

 

[Expidia]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Mine was 1.5 inches. Yours is probably different. Try raising it with a block of wood first of different sizes until you can view it from the side and the cav plate is around even with the boats bottom. The test drive it on the water. if good, then make up an aluminum bar. if prop is catching air lower it 1/2 inch at a time to see where you get a happy medium of less spray and better handling.

All boats have different hull design, so no one can suggest at what height is ideal with out test driving it. You have to tinker with it until you are satisfied. Most boat dealers are not going to invest the time to tweak it and water test it when you get a new boat. Many are not even near water!

My dealer when I bought my last new boat even had the boat wrongly positioned by about two feet on the trailer. I had to tinker with that too. Had to move the axle back a set of holes to get weight at around 10% on the hitch ball. This made the boat trailer a lot more stable on the road while being towed without bouncing up and down.

 

[pvanv]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Well, I didn't mean to "come back" with anything. I just assumed that you buy a 15" shaft outboard it's going to work with your boat with the 15" transom. . .

Luckily I have no bolt holes in my transom; I'll try to get a 1.5" aluminum block made and probably end up bolting it down to the transom instead of just using the clamps.

Thanks for the help

Wrong. The 15" shaft length is "nominal". Each and every rig must be individually setup and tested. Ditto for prop pitch. Each boat is unique, even "identical" boats. The ventilation plate needs to be about even with the bottom of the transom, then jacked up or down as required by the particular rig, and as determined by on-water testing.

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Interesting; I was unaware of this. . . .

I'll mess with the height and see what happens.

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

So I went out and measured the space. The A/V plate is 1.125" below the bottom of the keel, so maybe that's problematic but I also think I needed to trim it more.

I'll try a 1" block to see if that helps too.

Thanks everyone for your help on this.

 

[Expidia]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

So I went out and measured the space. The A/V plate is 1.125" below the bottom of the keel, so maybe that's problematic but I also think I needed to trim it more.

I'll try a 1" block to see if that helps too.

Thanks everyone for your help on this.

Trimming it in will only cause more spray and is used to get boat up on plane faster. Trimming it out once on plane will speed the boat up a little as there is less lower leg in the water. If you don't have power trim then you just have to experiment while on the water moving the trim holes until you hit a happy medium.

But I would start with the outboard height first. You may have to go an inch above depending on your hull design so now you have a range of 2.125 to experiment with.

Once the height is set then you can experiment with a different prop pitch. The prop the outboard came with if it's OEM (original equipment manufactured) is usually matched up to the O/B. Boaters that want to squeeze every extra MPH out of their set up will try a different pitch.

I actually went the other way, I carry a spare prop anyway, so I got one of a different pitch that I swap it out when I know I'm going to have 4 people on my small 15 foot aluminum. It gets me the extra ooomph I need to get it on plane faster. Other wise, I run with the stock prop which takes me to plane in about 5 feet with just the two of us onboard. I think I have a 13 and an 11.

I forgot which way the pitch goes up or down for when I have 4 people and I need more pull out of the hole shot. Plenty of threads on prop pitch on this forum or google it. Other wise I used to have to step out from behind the steering wheel and step a a few feet towards the bow while steering with one hand. Not the safest thing to do. Then while I was almost up on plane some boat traffic cuts me off and I have to go through it all again. The other pitch prop solved that for me with 4 people on board.

A dealer could have put the wrong prop on too originally if you bought your rig new, or a previous owner might have swapped a prop of another pitch, but that would not cause your drag if it's a few degrees off.

Get your motor height adjusted first. You won't believe the difference an inch or two can make once that cav plate is out of the water (I've been told that inch or two always make a big difference ) I had two huge rooster tails of spray coming off each side when mine was 1.5 inches lower! I used it that way from the dealer the first season cause I did not know any better until I read stuff on this forum.

 

[Sea Rider]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Yes trim engine/boat correctly and then try different transom shims untill you find engine sweet height spot. All short shaft engines are 15" but could have slight different small plate to AC plate heights between brands, a Yam probably will perform best than a Tohatsu, that's why you need to fine tune trim and engine heights for top performance as all hulls are not same. Dealers only sell engines, boats, whatever needs to perform as expected by owners is entirely your problem.

Happy Boating

 

[fishndirk]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Hello, I run the same exact motor on a 14 foot alumacraft flatbottom. I can tell you exactly what's going on with your rig just based off the video you posted. As others have said, your Anti Ventilation plate is sitting too low in the water. What's happening is its actually scooping up water and splashing it all over your transom. The small anti splash plate above the AV plate is suppost to take any small amount of water traveling up the foot of the motor and split it out to the sides but your AV plate is scooping up way to much water for it to handle that.
My 20 Tohatsu AV plate sits almost exactly even with the bottom of the transom and what happens is somewhere just above half throttle a little bit of water travels up above the AV plate but the small little anti splash plate splits it and it makes 2 little rooser tails straight out to the side of the boat but not a single drop splashes up past that the whole time.
If your boat is a 15 inch transom that could mean its anywhere between 14 and 16 inches in reality. From what you've posted I would say raise the motor up with a block atleast 1 inch, maybe 1 and a half and you should be good to go. By the way, I hope that the picture you posted is not how you trailer your boat or how you run it. Tilted down way too low. I tow mine so it sits straight up and down and strap the motor down so it doesn't bounce on the transom. With smaller motors like what we are running as long as you can keep it from bouncing on the transom while towing you will never have a problem with it. Hope all this helps.

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Thanks for the insight, fishndirk (and everyone else!); I trimmed the motor and it did run a lot better. Also I'm going to shim it 1" to see how that helps, too.

I was trailering/running the motor like that, yes. I'm an idiot.

The hole shot is definitely more sluggish but it planes better now and I'm not getting splatter all over the place. Still, I think a shim will do wonders for it.

 

[duke33]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

All this is good to know. Thanks everyone.

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

So I shimmed up my motor about an inch; I'm going to try it out tonite and see if performance improves.

Let me know what you guys think if this looks okay. I don't want to "do it wrong."

https://sphotos.xx.fbcdn.net/hphotos-ash4/409631_10150998238786730_863420910_n.jpg

 

[BonairII]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

The shim looks fine.

It's all the water that's coming out the drain hole that concerns me. Does your boat take on a lot of water regularly?

 

[catfighter]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

The shim looks fine.

It's all the water that's coming out the drain hole that concerns me. Does your boat take on a lot of water regularly?

No; I forgot to take the plug out and it rained.


I ran the boat for a solid hour and maybe a half-gallon of water was in the bilge area.

 

[Expidia]

Re: Boat motor seems to make a huge wake . . . Tohatsu 20hp

Looks better but a tighter shot from the side showing where the cav plate now resides as to the boats bottom would help. No big deal though as only your test drive will find out the higher position improves the spray. it will!
PS: don't forget to put that plug back in.