Honda BF20 short shaft doesn't fit a 15" transom

[Cobra819]

Bought new a BF20 short shaft for my 14' aluminum boat in ABOUT 2005. my boat has a 15" transom, so I knew I needed a short shaft. Simple, right? NOT! hung it on the boat, went for a test drive, and huge spray and backflow of water into the boat. After messing around AND calling the dealer to confirm short shaft via the Serial Number, I figured it must be the boat. I inserted a series of test shims until the problem was corrected, then took the boat to my aluminum guy where he added to the transom (TIG'ed a piece on to the top of the transom, did a great job). everything was fine, although the boat itself was narrow and quite a handful at speed.
I want to upgrade my boat, so knowing much more now than I did when I bought the Honda, I measured from cavitation plate to transom mount, and came up with 17 1/4". I even hung the motor on a wider Lund 14' to test (I thought maybe the weight of the motor on a lighter boat was causing the motor to sit deeper in the water), but the same backwash occured.
So the motor clearly doesn't fit a 15" transom (even though 15" - short shaft is the match), and will sit too shallow on a 20" transom, so what is my solution? I am certainly not going to buy a new boat for $3000 and modify the transom again to fit.
Thanks for any and all help in advance.

1977 21' Bayliner Admiralty
1977 14' Starcraft V5 - 60 HP Evinrude
1985 14' Naden - BF20 Honda

 

[Sea Rider]

As you have experienced not all 15 shaft engines will fit a 15 short shaft transom as a glove. Read this to understand your issues.

http://forums.iboats.com/forum/engin...transom-height

Post a pic of your set up showing engine sideways, want to check something to give you some hints to improve your current engine/boat installation.

Happy Boating

 

[Cobra819]

No Title

Thanks for the reply Sea Rider.
Here are a couple of pictures with my current setup showing the transom addition and the cav plate position.
Again, I am happy with this setup, but I want to upgrade my boat without modification. How can I hang this Honda on a new transom without mod?

 

[Cobra819]

No Title

Thanks for the reply Sea Rider.
I'm happy with my current configuration, but I want to upgrade the boat. According to your attached article, Honda is best 2" below the transom, which would account for my 17" AV to top of transom measurement. However, the 2" below is too deep and causes backwash. the motor performs perfectly on a 17" transom (shown here), but boat manufacturers don't make this transom.
How can I hang the motor on a new boat then?

 

[Sea Rider]

So you want to upgrade to a new boat with same current engine without messing with the transom height, right ? If so, you`ll be extremely lucky to make a sweet engine/transom match. What`s stated on my post regarding Honda, is plain theory extracted from Honda Owner's Manuals. Not always works to customer satisfaction as boat's hull and transom height has not been taken into consideration whatsoever. If wanting superb boating you must match engine height to achieve water flow passing slightly under upper water delector plate. That's with weight evenly distributed on deck and engine sitting at 90 angle on transom. Check your modified pic.





If you need to raise transom or chop a bit it's other awfull story. It's much preferably for water flow to pass under small w.deflector plate that over it, if over will give out or over water splashes that will slow you down.

Love doing engine/transom optimization or fine tunning Tohatsu/Nissan S & L engines to match 15.15.5-17-18-20 transom heights. All are dialed for water flow to pass slight under water delf plate.That's the ideal lower leg height for any brand engine.

Happy Boating

 

[Cobra819]

Thanks Sea Rider, I always thought transom height was pretty cut and dried, I never thought that mods were required (i've never seen it up here at Lake of the Woods in Canada). Seems like my best and neatest route would be a jackplate style adapter for outboards. I did some research and found a couple of nice manufacturers on the net. Do you have experience with these little ones meant for outboards? There are various setbacks starting at 4", seems like that would be fine.
Thoughts?
Thanks again for your advice btw...

 

[TwoRivers]

Thanks Sea Rider, I always thought transom height was pretty cut and dried, I never thought that mods were required (i've never seen it up here at Lake of the Woods in Canada). Seems like my best and neatest route would be a jackplate style adapter for outboards. I did some research and found a couple of nice manufacturers on the net. Do you have experience with these little ones meant for outboards? There are various setbacks starting at 4", seems like that would be fine.
Thoughts?
Thanks again for your advice btw...

Nothing cut and dried when it comes to boats or motors.
Builders and manufacturers make it up as they go.
Make or buy a jacker and set your motor where it's supposed to be.
good luck
rob

 

[Sea Rider]

Rob,

Perfectly in agreement with TR, before you invest on a transom mount, jack plate need to determine at which tail height is water flow passing by. That's how much you need to raise transom or engine. Go for a wot test with new boat and check, post results.

Happy Boating

 

[steelespike]

So you want to upgrade to a new boat with same current engine without messing with the transom height, right ? If so, you`ll be extremely lucky to make a sweet engine/transom match. What`s stated on my post regarding Honda, is plain theory extracted from Honda Owner's Manuals. Not always works to customer satisfaction as boat's hull and transom height has not been taken into consideration whatsoever. If wanting superb boating you must match engine height to achieve water flow passing slightly under upper water delector plate. That's with weight evenly distributed on deck and engine sitting at 90 angle on transom. Check your modified pic.





If you need to raise transom or chop a bit it's other awfull story. It's much preferably for water flow to pass under small w.deflector plate that over it, if over will give out or over water splashes that will slow you down.

Love doing engine/transom optimization or fine tunning Tohatsu/Nissan S & L engines to match 15.15.5-17-18-20 transom heights. All are dialed for water flow to pass slight under water delf plate.That's the ideal lower leg height for any brand engine.

Happy Boating

I think you miss spoke. "motor 90 degrees to the transom"Would take the prop out of the water .Deflector plate 90 degrees to the transom would put the
the motor about 10 degrees off of 90 degrees to the water surface.
90 degrees to the water is probably good for 90% of applications. I say trim to the best performance.
Most of the boats manual trim I have done usually end up in the 2nd notch out 3 being too much and one not enough.
Many times I think 2 1/2 notches would be ideal.

 

[Sea Rider]

Not all transoms angles are built same, that's why some engines needs to be adjusted on 1-2-3 trim hole out transom, usually it's second hole. Try to match a hole to have motor sitting at 90 deg (perpendicular) to water level. There's an issue here specially with under powered or portable engines. If AV plate, prop shaft does not run parallel to water surface when boat is on plane will lose best prop thrust...To optimize an engine to sit at the Sweet Heigh Spot is a complex art..

Happy Boating