Transom Angle and effect in Outboard Trim

[Andrew Leigh]

Morning all,

I have a 17' RIB with 2 x 40HP Yamaha's (South African Rib design). I have been unable to trim the bow up sufficiently as to optimize the trim fully. I get excessive spray from the sides of the boat and one can "feel" resistance, I am unable to cavitate the props using the trim switch, all my other boats I have been able to lift the motors sufficiently the motors will cavitate. I normally will trim until I hear the first signs of cavitation and then back off a little, take a hard turn and listen for cavitation, if none leave it set right there or back it off a little more.

I pop onto the plane really quick, but the trim reaches its extreme limit, and the motors will not raise any further when the hydraulics move into tilt phase. I recently established (after much head scratching) what I believe to be a manufacturers error that I now need to correct. Old boat, no recourse.

The motors trim range is -4° to +20°.
The transom is -15°

This means that the transom angle has utilized 15° of my trim leaving me with only +5%. For this reason I believe, I am not able to fully optimize my trim when on the plane.

The only solution I can see is to purchase and fit transom wedges. I was thinking that a +5° wedge would do it as I am very close to perfect trim with the +5° currently available to me. In fact, I think that even +2° would suffice but I don't want to chance it without some science proving it.

Is there math to prove my assumption correct?

Secondly, one can over trim the motors negatively, in my case this would -19° of trim. I have noted that the motors stall when fully trimmed in as I think the float levels are adversely affected. I would like to fit trim pins to avoid over trimming into the transom. What should the motor attitude be when trimmed for getting the hole shot 0° or must it be in negative territory?

 

[baeckmo]

I'm afraid you're barking under the wrong tree. Standard transom angle for outboard and I/O is 12 degrees (some claim 13), with a tolerance of about +-2 each way. This means that the mounting bracket "neutral angle" is 12 degrees, from which you add or detract your trim. Before digging deeper into this case, I'd like to see some pic's on the installation and info on the propellers and the boat dimensions.

Beware that it is not uncommon to find that aft sections of RIB collar are "riding" on the surface, effectively preventing the bow to trim up, no matter how you trim the engines. Your comment about "excessive spray" is indicative here.

 

[Andrew Leigh]

I am happy to receive correction.

The transom is 15° by measurement.
RIB length is 5.2m
Beam 2.3m
Hull dead weight 350kg
WOT is 5 500rpm
Propellers are 11 and 1/8 x 13 - G
Boat runs in salt water.
Outboards are Yamaha 40XWT

Side view

Tomorrow I will remove the boat cover to take more detailed pics. I assume you will want to see the cavitation plate with reference to the bottom of the hull as well as detail about the mounting bracket.

 

[Cajunpockettunnel]

I am happy to receive correction.

The transom is 15° by measurement.
RIB length is 5.2m
Beam 2.3m
Hull dead weight 350kg
WOT is 5 500rpm
Propellers are 11 and 1/8 x 13 - G
Boat runs in salt water.
Outboards are Yamaha 40XWT

Side view

Tomorrow I will remove the boat cover to take more detailed pics. I assume you will want to see the cavitation plate with reference to the bottom of the hull as well as detail about the mounting bracket.

I am happy to receive correction.

The transom is 15° by measurement.
RIB length is 5.2m
Beam 2.3m
Hull dead weight 350kg
WOT is 5 500rpm
Propellers are 11 and 1/8 x 13 - G
Boat runs in salt water.
Outboards are Yamaha 40XWT

Side view

Tomorrow I will remove the boat cover to take more detailed pics. I assume you will want to see the cavitation plate with reference to the bottom of the hull as well as detail about the mounting bracket.

It appears to me your engines are too low. At least that is what it looks like according to your pictures. I could be wrong but, raising the engines a couple bolt holes is where I would start.

 

[Andrew Leigh]

Tomorrow I will post a sketch of the mounting details, including motor height relative to the hull.

 

[Cajunpockettunnel]

After looking at the pictures of your rib, it definitely appears your engines are too low. The cavitation plates need to be flush with the bottom or a little above. That will give you more trim and less drag.

 

[Andrew Leigh]

Thanks, let's see what the measurements show. Out of all the permutations this was not one I had considered. If you are correct then I can go up a hole or two, but I cannot go down.

 

[Cajunpockettunnel]

Thanks, let's see what the measurements show. Out of all the permutations this was not one I had considered. If you are correct then I can go up a hole or two, but I cannot go down.

Use a straight edge. Try to flush the cavitation plate to the bottom of the hull.

 

[BlueBell]

Not sure what your worry about transom angle is, you can't change it.

Raise the engines up until the AV plate is flush with the hull bottom when trimmed parallel.
Or higher if there's no bolt hole that lines up within a 1/4".
No picture will be as accurate as a straight edge extended off the hull bottom.

 

[philSweet]

Guys, he trying to get the bow UP. If anything, lower the motors and block them aft a few inches if the harness and cables will stretch. But it probably won't help overall. Getting rid of some forward weight might help.

Of course you can change the transom angle. That's what mounting wedges are for.
Robot or human? https://www.walmart.com/ip/5-Degree-Positive-Tuck-Transom-Wedge-Port-Starboard-1-Set/13932710189?wmlspartner=wlpa&selectedSellerId=101001238&selectedOfferId=598A7A846099349F9277F14DE55F1163&conditionGroupCode=1

 

[BlueBell]

You know best philS, you take it from here.

Good luck Andrew. You may want to take your phone numbers off your photo, your call.

 

[Andrew Leigh]

You know best philS, you take it from here.

Good luck Andrew. You may want to take your phone numbers off your photo, your call.

Oops did not notice that, thanks. Edited.

This side of the pond skippers are required by Maritime Law to add our telephone number and alternate contact number to the trailer. The theory is if they see an empty trailer, they will call, if no reply they will be alerted to mount a search.

 

[Andrew Leigh]

Ok.

The got the hull level using a spirit level, adjusting the jockey wheel upfront.
I then trimmed down the one outboard until the AV plate was also level using the same level.
I then marked the height at which the AV plate would intersect the transom.
The drawing is to scale.
Thought I could not go down but I can, I have one hole to go so I can go down by 18.5mm (0.73") or up by 2 holes 18.5 plus 18.5mm.
On the Yamaha rigging manual, the first dual outboard with a specified distance between motors is a 55hp and they say 660mm. I am 100mm shy of that?

 

[Andrew Leigh]

Guys, he trying to get the bow UP. If anything, lower the motors and block them aft a few inches if the harness and cables will stretch. But it probably won't help overall. Getting rid of some forward weight might help.

Of course you can change the transom angle. That's what mounting wedges are for.
Robot or human? https://www.walmart.com/ip/5-Degree-Positive-Tuck-Transom-Wedge-Port-Starboard-1-Set/13932710189?wmlspartner=wlpa&selectedSellerId=101001238&selectedOfferId=598A7A846099349F9277F14DE55F1163&conditionGroupCode=1

Almost all the weight is aft, motors, main fuel supply, and 260lb skipper. I have the anchor up front, the batteries are under the helm, reserve fuel (20kg) under the seat in front of the helm.

 

[Andrew Leigh]

I am being fussy, I like things to work properly. The RIB pops onto the plane almost immediately, the RIB planes well and handling is excellent. The RIB however in my opinion could plane more efficiently and I would also like to trim up a little more if sea conditions require it and for a following sea.