long shaft or short shaft on futura

[learning to fly]

I have a futura fr mk2c. year is 99. I used the boat this summer with a 9.9 mercury short shaft. It ran great. My son could plane with 3 150 lb friends in it. Really surprised me. I have a 15hp mercury that is a long shaft. I was wondering how this would affect the ride being 5 inches lower in the water. every once in a while I noticed the 9.9 would cavitate a little when I would be in good chop. Not much or often but it would happen. The long shaft would stop this Im sure, though this is not the reason for the 15hp. That is strictly more power That I am looking for. any thought on this, Also can I just switch lower units. Are they geared differently?
thanks

[Locozodiac]

Don't know about futura models, have seen Zodiacs "Classic model 350" with Yamaha 15 long shaft working fine so far, try your 15 LS and see how it performs, must work ok and it's not that big/powerful to mistreat the transom at all. Inflate the sib to it's correct working pressure. You could switch both gears cases and its related extension parts as long it's a disguissed Tohatsu brand as both models are geared the same 1:85-1, can't speak for the Yamaha, Mercury rebadged 10/15 models. If not mistaken, the MK2C (3.70 mts model) uses short shaft engines.

Happy Sibbing

[BumbleAbout]

See http://www.zodiacmarine.com/uk/the-zodiac-range/?futura (click on "Technical data") for the various shaft/ engine options. Both of the Mark 2C models appear to be short shaft (30hp or 15hp depending upon which floor option you have).

I'm afraid that I don't know if you can just swap lower units.

I'm running a 4.1m Mk 2 HD with a Yamaha 50hp longshaft and it flies - the cavitation plate on the outboard is about an inch above the lowest point on the transom (5 inches lower would not be good). I vastly reduced cavitation by adding doel fins to the outboard this year - they stopped the bow rising when accelerating to get on the plane and smoothed out the ride too (whilst losing some top speed). This helped pulling water skiers, so I'm not too worried about the top speed loss.

[jyasaki]

Originally Posted by BumbleAbout

See [url]I'm afraid that I don't know if you can just swap lower units.

You can. If you can find one of appropriate length. I'd also check that the cost is less than a comparable used motor of appropriate length (it may not be, especially if going for a new LU.)

The other option is to mount the engine 5 inches higher (have to make sure the added transom area is strong enough, though.)


jky

[Locozodiac]

QUOTE: BumbleAbout; I'm afraid that I don't know if you can just swap lower units.

If it were a Tohatsu 9.9/15 rebagded Mercury 2 strokes, you could swap complete gear cases as 9,9/15/18 models share the same gear cases including same gear ratio, with prob different prop size pitches; we do not import 9.9 /15 HP models to have a look at their mediun range size props between both. For the conversion, will only need to transfer the 9.9 shorter parts; extension housing, drive shaft, cam & shift rod and place it on the 15 to use as a standard 15 short shaft engine. Before messing around with both engines, better test your boat with 15 long shaft and see how it performs, if badly, as Jyasaki says rise the transom accordingly. It's almost normal to have/experiment some cavitation on choppy or wavy seas while cruising at high speed with short shaft engines.

A Yamaha 25 and not a 15 long shaft as stasted on previous post, was placed on a Zodiac Classic 3.50 inflatable deck model 2 years ago, correctly inflated still performing fine with no transom prob at all.

Happy Sibbing

[m chappelow]

apart from a bit more spray and a slight drop in speed it may be better with the long shaft when in rough water as the prop is less likely to leave the water causing it airate or cavitate ,the spray will come from the upper part of the leg where it flattens out a bit where the gear linkage is exposed . i got around that on mine by making a hard wood block shaped like the rest of the leg with a slot filed out for the gear linkage to move which makes that bit more streamlined .suppose one could be made from a piece of aluminium plate bent to to leg profile . i just strapped it on with 2 cable ties ,

[Locozodiac]

QUOTE: BumbleAbout; I'm afraid that I don't know if you can just swap lower units.

To end the rebadged brand guessing, is this the type of 9.9 & 15 HP 2 strokes engine you have right now ? If positive, is a Tohatsu and will give splashing problems if your transom is under 40 cm height. This is because of the square exhaust compared to other brands better shaped exhausts when meeting with tail. Is a matter of rising the transom a bit,

This problem is notorious in normal sibs, don't know their behavior on Futura, having extra tubes under the main tubes, wether it will rise more the sib’s transom once sib on water, compared to standard tubes sibs.transom’s. Test both under trial and decide.

Happy Sibbing

[m chappelow]

pic of the wooden profile used to stop spray that i use on my long shaft .

[learning to fly]

Thanks for all the help. Am I to figure that one of the things I would be most worried about when useing a long shaft instead of a short shaft is the additional force on the transom? Is this my biggest problem. The 15hp shouldnt be too bad. I guess a trial is the only way to be sure
thanks
scott

[m chappelow]

unless you start to get into the laws of leaverage and figures ect i dont think you will have any trouble, i have used long shaft engines on inflatables for well over 30 years and i have never had any problems with transom stress , theres only about 6 inches difference anyhow leaverage wise ,only problem that i have ever had is on some engines they were a bit harder to start owing to the bit extra back pressure .and the risk of touching the bottom a bit earlier .

[Hampton Pete]

I am using a futura 2c with a 15hp short shaft as its only 15", as recommended by zodiac,,,,my yam longshaft is 23"......surely thats too long for safety?