Transom spacer

[Koldo]

Hello everyone,

I am new here on the rib.net forum and it is really a great source of information!

I got myself a used Tohatsu 15hp for my older hard floor SIB. Up until now I used old Yamaha 4hp 2-stroke, so you can imagine my joy from upgrading!

The boat planes very well, the top speed with 2 people onboard was 25.5mph. Overall it feels great, but there is one thing that bothers me a little - the leg of the outboard causes a bit of splashing. I went through several threads and from what I read I assume it might be caused by the height of transom and lenght of the outboard leg. The AV plate sits arround 1” below the transom level, which isn’t too much, but probably not good enough.

I am thinking about trying to raise the outboard a little by using a spacer. However, I am not sure what that spacer should be. Saw some pics of wooden spacers. Any other ideas? Should the spacer be attached to the transom or the weight of the OB is enough to pressure it down as long as I tighten the screws enough?

Adding also some pics of the splashing water, not the best quality due compression for the website, but you might get the idea.

[Peter_C]

Welcome to RIB.net

The engine looks like it has a pretty good tilt to it. Before you do anything have you tried tilting the outboard up a little, like one hole? If it doesn't plane as well and the bow comes up it needs to go back down a hole. If you have a manual tilt that locks you can accomplish the same thing without moving the pin.

Splashing is pretty common and not something to be really concerned with. You could throw a block of wood under the outboard and take it for a quick spin to see if anything changes. The cavitation plate being 1" lower is about right. Much higher and the prop might start cavitating.

I would recommend adding a leash to your outboard attached to a solid mount on the transom. Many an outboard has been lost...overboard

[Fenlander]

Hello and welcome to the forum.

Agree with Peter's comments. All I can add is that I've found different make/model outboards vary a great deal to how much splash they generate. Usually for trials we just slip a wood block under the outboard saddle not fixed to anything. You will see how it works out on your SIB but once your shim goes above 25mm you might start to lose enough engagement of the clamps.

[Steve509926]

Hi and welcome koldo.

When you say a bit of splashing, is it coming back over the transom and into your sib?

[jonp]

I normally have the plate you mention around 1inch above the bottom of my boat to reduce drag and increase performance. This often means fitting at the highest mounting holes on the outboard. You haven't provided a picture of the back of the boat to show if there's an option to mount higher. There are various brackets, wedges and jacking plates available for mounting engines too. Ive currently gone from a long shaft to extra long on one of my boats using a bracket that allow me to raise the engine a huge amount compared to outboard normal mounting holes.

As you can see in this image, there's not really a right or wrong height, it's more about what performance you are trying to achieve. The lower the engine sits in the water the more drag it makes, meaning more fuel and less speed.

[Steve509926]

Originally Posted by jonp

I normally have the plate you mention around 1inch above the bottom of my boat to reduce drag and increase performance. This often means fitting at the highest mounting holes on the outboard. You haven't provided a picture of the back of the boat to show if there's an option to mount higher. There are various brackets, wedges and jacking plates available for mounting engines too. Ive currently gone from a long shaft to extra long on one of my boats using a bracket that allow me to raise the engine a huge amount compared to outboard normal mounting holes.

As you can see in this image, there's not really a right or wrong height, it's more about what performance you are trying to achieve. The lower the engine sits in the water the more drag it makes, meaning more fuel and less speed.

jonp I think you may have missed that it's a 15hp Tohatsu hung off the back of a sib.
Talk of brackets, jacking plates, wedges etc is probably a bit ott for what the OP mentions "a bit of splashing"

[Koldo]

Hi everyone!

Thank you for all the responses.

I have also considered the option that splash might be just normal, something what comes along with bigger outboard, but wasn’t sure. The rear of the boat and the motor got wet and it was sprinkling slightly over the transom too. Not something I coudn’t live with, but it’s getting colder and I prefer to stay dry

I haven’t tried tilting the OB yet. The boat was planing quite level and it might be rasing the front. But I will give it a try for sure. If that doesn’t help, I will try to add wood spacer.

Already have a provisional leash there. I had to reglue the transom to the floaters and I am a little paranoic about it. I already started to play with a thought of buying a bigger SIB, something like T38, but maybe next season

[Peter_C]

That little red curly cord is VERY important to attach to the boat driver. Many a video has been made of boats without drivers, which is even more common with SIB's as they are easy to fall out of. Around the dock is when most don't wear their kill cords and just happens to be when most incidents occur. Many countries around the world now require the kill cord be worn at all times.

[Koldo]

Originally Posted by Peter_C

That little red curly cord is VERY important to attach to the boat driver. Many a video has been made of boats without drivers, which is even more common with SIB's as they are easy to fall out of. Around the dock is when most don't wear their kill cords and just happens to be when most incidents occur. Many countries around the world now require the kill cord be worn at all times.

I thought about it beforehand, but as I haven never used it with my 4hp (never planed), I initialy forgot to attch it to myself. Later I realised and clipped it to my life jacket. I need to make it a habit, just like seatbelt or helmet on a bicycle