Engine height

[IanH]

I guess from the height of the cavitation plate bottom of transom, my engine is a couple of inches too low? Image imported upside down by the looks of it.

Looking at engine mount, I've probably got enough adjustment to bring it within an inch, but do I put anything under the engine mount, like a strip of wood to take the weight of the engine off of the bolts and onto the top of the transom? Image seems to have rotated 90 degrees on import.

Not sure why the images are rotated, but you get the picture.

[Locozodiac]

Which engine brand is it, is it a S or L shaft leg, with ? HP mounted in which type of boat, sib, rib ? Post a complete pic as first one, but showing the prop too. Correct inversion as not to turn my monitor up side down...

Happy Boating

[IanH]

Zodiac 3.6M Fast Roller with Mariner 9.9HP (4 stroke) short shaft (approx 36KG)

Images annotated and corrected hopefully

According to engine manual, I need to get cavitation plate within 1" (25mm) of bottom of transom. So probably need to raise engine 1 to 1-1/2" above height of transom, which is probably within the level of adjustment allowed by the mounting plate on the transom?

If I had the choice, do I edge nearer cavitation plate to 1/2" below bottom of transom or stick to 1".

But this will leave engine aloft of the transom, so I guess it is wise to put something beneath it and top of transom to support it? Any suggestions? Strip of 35mm wood is a bit heath robinson. I nice block of soft acrylic plastic would probably do the trick nicely - of course, someone may sell these types of things.

[Nos4r2]

I think you're ok there-the tubes are lower than the bottom of the transom.

[IanH]

Thanks. That makes sense.

I'll check the offset with a straight edge below the tubes.

[jyasaki]

I'd try to run it higher. May take experimentation, but some boats run the plate up to an inch above the keel level. Higher is generally better up to the point where you get ventilation. As you get close to ideal, it's a balance between drag and splashing on the low end, and blowing out in turns at the higher end.

I don't think the tubes really come into the equation. Has to do with how the motor sits on the water as it passes off the bottom of the hull.

jky

[IanH]

I measured level against bottom of tubes. Cav plate was higher than bottom of tubes. So cav plate is below transom but above bottom of tubes. Not sure where waterline is.

I've got quite a bit of running in where I can examine levels in water. For time being, I will assume that Zodiac made transom correct height for their short shaft engine requirements.

[willk]

Ian - stick a bit of 2"x1" in there and try it out. Works for me.

[Locozodiac]

Anticav plate/boat's bottom height parameters are just a refference from where to start testing under trial and error as correct engine/transom height match will depend entirely on boats bottom shape, tube diameter, correct pressure on air deck & tubes, brand engine used.

Try this simple guide :

-Inflate air deck and tubes to factory working parameteres, usually 3.0 PSI minimum and evenly for tubes, 12 PSI for air deck.

-Sit engine trimed so to have a perpendicular engine and anticav plate paralell to water level once sib is floating. Test on flat calm no wind water condition preferably.

-To ballance well sib, sit a mate at bow, driver must seat on air deck next to transom. Use your kill cord and hold tiller firmly to go on a straight water course when at wot.

-Check water flow passing at speed through lower leg. Ideal water flow height to have excellent hole shot, superb tight turns and good top speed is achieved when flow passes slightly under small upper deflector plate.

By visually checking this flow issue, will know for sure if in need to raise the engine and by how much if needed. Test with different wooden shims thickness untill you achieve the described sweet engine/transom height match position.

Check this video for clear understanding :

Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Happy Boating

[Locozodiac]

Permanent Modified Engine Heights Examples :

Most sib/rib boaters assumes that all boats are factory deliverd with correct transom heights that will perfectly match any brand engine. But this sometimes isn't so, if you have the time and patience a wot spin under trial and error to achieve best engine/transom height match is mandatory.

Problem : Excessive water splash over transom at speed.
Boat : Sea Rider 380 sib/Tohatsu 18 S.
Tech issues : Transom being 38 cm high when needed to be 40 cm.
Correction : Raised transom to 40 cm to find sweet engine /transom height match.
Performance : Engine has gained extra 250 RPM , has perfect hole shot, excellent close turns at any speed, better fuel economy needing to throttle much less to maintain plane.

Happy Boating

[IanH]

Thanks. That's really helpful.

Just two questions...

What is 'hole shot'
What is 'perfect hole shot'

[Locozodiac]

Problem : Excessive Prop Aireation at turns & choppy seas
Boat : Sea Rider 420 Rib/Tohatsu 18 S.
Tech Issues : Transom being 45 cm height when needing to be 40 cm.
Correction : Chopped transom down 5 cm to find sweet engine/transom height match.
Performance : Engine runing at near it's max wot rpm parameters, excellent hole shot, perfect close turns at any speed and water condition, better fuel economy needing to throtle less to maintain perfect boat ballance & plane.

Happy Boating

[Locozodiac]

Originally Posted by IanH

Thanks. That's really helpful.

Just two questions...

What is 'hole shot'
What is 'perfect hole shot'

Hole shot :
When boat goes from a steady floating position trying to pull out of water, be on it's way, when at least medium to full throttle is applied.

Perfect hole shot;
Would be trying to get out the fastest as possible to instant plane.

A perefect engine/transom height match achieves fast hole shots including the use of portable & medium HP engines delivered with standard props mounted on light sibs/ribs.

Happy Boating

[jyasaki]

Hole shot is the boat climbing up onto plane. Takes more power to get itup on plane than to keep it there.

jky

[9D280]

Just one wee observation -

If I'm reading those pics right, is that engine only held on by the clamp screws? (i.e not through bolted between clamps & transom?)

If so, keep a good close eye on it while testing if you lift it! - at least your clamp plate has a ridge. The rest you can probably work out!

[IanH]

Its not even got wet yet! The wheels and engine had been fitted about 5 minutes when pictures were taken.

Planning to start using it over half term week so just a bit of forward planning for then whilst I have time to ask. With running in and probably sticking to river, Im not even expecting to try and get it on the plane until I get the conditions and confidence to run the gauntlet of the harbour mouth.

[steco1958]

Originally Posted by willk

Ian - stick a bit of 2"x1" in there and try it out. Works for me.

Always worked for me on my Futura, make the adjustment you will get a feel for when it is correct.

Steve

[Locozodiac]

Bolt engine to transom if you like after ideal engine/transom heigh has been reached, not before. If raising engine will need to raise both rear & front engine plates accordingly to itos new position.

If possible glue a piece of pvc, haypalon, tire blader inside metal plate. Adjust both clamps tight against. If eye clamps matches facing each other, tie a small rope between them, so engine is secure. I've seen boaters droping to sea their portable engines that were only clamp tight which came lose due to engine vibration issues while cruising.

Happy Boating

[IanH]

Thanks to photos posted on here and most excellent performance that I witnessed today, I don't think that I will be experimenting with engine height. No/minimal splash back, water level just below upper plate combined with ease of plane and ability to maintain plane whilst throttling right back.

Thanks to all that contributed especially locozodiac for his photos and indepth descriptions.

NB... Engine was tied on!

[Locozodiac]

IanH,

You are welcome, nice to read that pics info worked for you. If slight under or bit under small upper plate, that's perfect. The big bummer is when having over transom water splashes, Tohatsu leg shape issues.

Happy Boating