Dink OB - Long Shaft vs Short Shaft

[colemj]

Originally Posted by Terra Nova

Delusions of grandeur.

A lot of your contributions to this forum consist of you spouting some unexamined belief, someone else disagreeing with it while laying out their reasoning and experience, and you coming back with only an ad hominem attack on them.

Several threads I have asked you to describe your specific experience and reasoning regarding some belief I didn't agree with and you got snotty over - and each time you ran away from the thread.

Don't be a coward. Stand here and explain your reasoning.

You might just change my mind about the topic. All I have to go on is a lot of experience with too long/too short legs on small runabouts, coupled with a bit of brain noodling around the issue.

I've laid both of those points out - but I am still willing to examine alternate reasoning.

Something you seem to have a closed mind to.

Mark

[Terra Nova]

Originally Posted by colemj

...Stand here and explain your reasoning...

Do your own due diligence. You are the one recommending the use of a long shaft motor on a dinghy designed for a short shaft. If your only argument is that is how you do it and no one has died, it's not a good enough reason to go against the dinghy manufacturer's recommendation and common sense.

As far as claiming you have more experience about these things than the rest of us...

[colemj]

Originally Posted by Terra Nova

Do your own due diligence. You are the one recommending the use of a long shaft motor on a dinghy designed for a short shaft. If your only argument is that is how you do it and no one has died, it's not a good enough reason to go against the dinghy manufacturer's recommendation and common sense.

As far as claiming you have more experience about these things than the rest of us...

As I expected, you will only denigrate and not back up anything you post.

And a poor reader. If you go back to my posts, you will see that I recommended following manufacturer's recommendation for the specific dinghy.

Where you were wrong about the above is saying that a short shaft is the only real answer. The correct answer is the one I gave - it depends on the dinghy.

I then proceeded to relate experiences I have had with OB's that did not perfectly match the dinghy - as well as lay out some reasoning regarding that "very critical" 5" some are wringing their hands over.

There is no death trap here. There is no safety issue of any practical importance. You might suffer a drop from 20kts to 19kts on your top speed though.

It is actually a larger problem having too short an OB shaft on a boat.

Mark

[44'cruisingcat]

Originally Posted by a64pilot

If you have a planing hull, the anti-cavitation plate should ride right on top of the water, you should see the top of it, dry when on plane.
Deeper than that and you lose significant performance, sort of like having the weight and fuel burn of a 15 HP, but the output of a 9.9. If your cool with that, then fine no problem.
But do make sure it's not a high thrust motor, they turn a larger prop at lower RPM to push a big boat, and while they will work too, their performance loss will be even greater.
I'd opt for the "proper" length motor, the one that both the boat and engine builder recommend. But then I'm also the type to "prop" even a dink, that is spend money to ensure the motor will reach full RPM but no more

LOL! More like having a 15 and the output of a 14.75.... or put it this way,
even with the "wrong" shaft length or 3.5m RIB with 18 hp, outperforms a similar RIB with 15, and the right shaft length.

I'm not saying we don't lose anything, but it's not much.

And forgive my curiosity, but how do you check the RPM you're getting? Does your dinghy have a tachometer?

[colemj]

I have OB manuals for a Honda 15hp, a Suzuki 20HP and a Tohatsu 18HP on board.

Regarding mounting height, all three manuals have specific warnings about the danger of mounting too high so the cavitation plate goes above the water. The only mention of mounting too low is that it may slightly decrease the top speed - no warnings boxes about this like for too high.

On our Achilles dinghy manual, they have specific warnings about adjusting the height too high because the dinghy will slip in turns and be dangerous. All it says about the leg being too low is that it might splash some water over the transom.

Do all of you worrying people only use your perfectly matched OB height on flat calm waters? Because if you use your dinghy in areas and conditions that we do, your cavitation plate will constantly be aerating and causing over-revving if it is mounted so the top of it is dry on smooth water plane.

Also consider the change in OB height as the dinghy is loaded.

Do all of you hand-wringers actually change the mounting height to match the weight of people or cargo the dinghy has on each trip? That could easily make your 15" shaft sit as low as a 20" one - and be imminently dangerous, of course…

Mark

[44'cruisingcat]

Originally Posted by Terra Nova

Do your own due diligence. You are the one recommending the use of a long shaft motor on a dinghy designed for a short shaft. If your only argument is that is how you do it and no one has died, it's not a good enough reason to go against the dinghy manufacturer's recommendation and common sense.

As far as claiming you have more experience about these things than the rest of us...

Why lie? Nobody is RECOMMENDING the use of a long shaft motor on a short shaft dinghy. All we're saying is that it will still work, it is safe (your comments about it being unsafe were frankly, complete and utter ********.) and that the loss of efficiency is not major. I doubt it would even be noticeable unless you ran comparisons with a GPS.

[colemj]

Since some here have reading comprehension issues, I would also like to reiterate what I posted earlier - that the worse that could come from this is that the OP may need to install a transom riser. These are cheap and easy - it could be as easy as adding a wood batten to the top of the transom, but they do make specific risers.

Mark

Mark

[Terra Nova]

Mark--you needn't be guessing at all this or making stuff up. The Achilles brochure specifically states standard length (15") shaft after each hp listing, until you get to page 12, where models intended for long shaft motors are listed.

http://www.achillesboats.com/pdf/fil...atalog2015.pdf

[44'cruisingcat]

Do you know the difference between a brochure and a manual?

[colemj]

Originally Posted by Terra Nova

Mark--you needn't be guessing at all this or making stuff up. The Achilles brochure specifically states standard length (15") shaft after each hp listing, until you get to page 12, where models intended for long shaft motors are listed.

http://www.achillesboats.com/pdf/fil...atalog2015.pdf

Wow, your reading comprehension is atrocious.

My very first post to the OP stated that his 9'10" Achilles recommends a 15" shaft, while our 11'6" Achilles recommends a 20" shaft.

I then spent time in other posts pointing out that dinghy manufacturers list the shaft lengths that should be used with each model.

Where you and I began to disagree is when you agreed with the categorical statement that all dinghies should have a short shaft. I posted that it depends on the specific model of dinghy and that there is not a hard and fast answer like you believe there is.

Subsequently, I pointed out that if the OP is getting a screaming hot deal on a 20" shaft over a 15" one, that the worse that would happen is he may have to use a riser on the transom.

You shouted death and destruction, and came up with some of the silliest and indefensible statements on this thread. I bet you can even come up with some danger doozies about using a transom riser?

Am I using too large of words?

Edit: are you hung up on the term "standard"? If so, you should be aware that it refers only to an agreed industry base term, and not that anything different is "non-standard", or a 15" OB is the "standard" one that should be used on all dinghies.

For example, a 20" shaft costs more. An electric start cost more too. Does having a "non-standard" electric start on a dinghy also cause death and destruction? Whoa be those who put on a "non-standard" prop instead of the "standard" version. How about remote steering over the "standard" tiller arm?

Do you think one size prop also fits all dinghies?

Mark

[a64pilot]

Originally Posted by colemj

That link refers to a 50mph boat running two 200hp engines. In this application, the engine height and attitude is always matched perfectly to the boat. In fact, the engine mounts themselves have adjustments to them for this purpose.

For the typical dinghy application, this is not important. If it was, manufacturers would be providing adjustable mounts because there is a very wide range of possible dinghy model transom heights and outboard mount combinations.

It is also interesting to note in that link that raising the engine mount so the cavitation plate moved from below the surface to at the surface gained them 1.5mph on their 50mph boat.

Mark

They were also I believe 5/8 an inch low, not 5 inches. Point of the link was to illustrate proper height, it had a picture, not anything else.
The adjustable plates you refer to are called Jack plates, Last Jack plate I had was hydraulic and adjustable at the helm, you had to start out with the motor dropped or the prop would "blow out". I've been away from those high horsepower go fast boats for awhile, but I've not seen a manufacturer put a Jack plate on their boat, dealers yes. but manufacturers DO provide adjustable mounts, every motor I have ever seen has a row of holes to raise or lower the motor, and another row of holes to adjust the tilt . These adjustments do have an effect, often a substantial effect, many times allowing a smaller motor properly set up to perform satisfactorily. Often times it's the difference between planing and not. Now you don't see me advising people on how to set up their sails etc., but motors and power boats I have a little knowledge in.
Ask yourself this, if length doesn't matter, then why are they available in different lengths?

Now if you just want to putt putt around and or have to use your big boat motor for the dink for financial reasons, sure get a long shaft, just don't be surprised when a 9.9 outperforms your 15 though, cause it will.

I got almost 5 kts out of "propping" my motor and another 3 from getting it to the right height, needs to go a little higher actually, but hey it is a dink it's fast enough.


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[a64pilot]

Oh and I will come up with a caution about a transom riser, it had better be pretty strong, not just a piece of wood to raise the motor. I need, want to make one myself, I'm thinking a couple SS plates welded to a SS square tube, plates extending down on either side of the transom with thru bolts through the transom.
Mark, you at your boat? We have the same motor, how wide does the riser need to be, 8 "?


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[44'cruisingcat]

Originally Posted by a64pilot

Now if you just want to putt putt around and or have to use your big boat motor for the dink for financial reasons, sure get a long shaft, just don't be surprised when a 9.9 outperforms your 15 though, cause it will.

And yet a 15 didn't perform as well as our 18?

[txh2oskier]

Originally Posted by 44'cruisingcat

Maybe you need to go back and actually read the replies.


Nobody has criticized you. What has been said is that the best option would be the correct shaft length, but if it's not readily available, or affordable, ie if the long shaft motor is a bargain, it will still work, so don't just discount it out of hand.

Yesterday I was unable to get "Quote" to work so it may have appeared my response was aimed at you. It was not. colemj said the original poster (me) was lazy and bought the wrong dinghy or wrong outboard. He needs to go back and read my original post.

Enough said.

[Terra Nova]

Originally Posted by colemj

...Where you and I began to disagree is when you agreed with the categorical statement that all dinghies should have a short shaft...

Actually where you and I began to disagree was way before this thread.

If I specifically said, anywhere, that all dinghies should have a short shaft, I'm sure you would have quoted it.

As far as, if the long shaft motor is too long for your dinghy, but is a great bargain, then it's OK, it seems a pity that all that experience you claim to have is not tempered by a tiny bit more common sense.