Positioning outboard on a pontoon

[JohnMarc]

This is a tough one to explain when firstly I am not by any stretch of the imagination an experienced boat builder and secondly I do not know all the correct terms....but my question is regarding the position of the outboard.
I see with a lot of the pontoon boats the outboards transom seems to extend further than the stern of the two pontoons. (I really hope I have explained it properly) is there an advantage to this besides possibly giving more deck area? I am busy designing a pontoon boat and have limited space in my small boat harbour on the Island and would prefer to have the outboard in line with the stern of the two pontoons. I have attached two photos hopefully demonstrating what I am asking.
Many Thanks in advance

 

[DogCavalry]

In the second image there's not enough space for the outboard to steer.

 

[JohnMarc]

In the second image there's not enough space for the outboard to steer.

yes I am aware of that..... I slid the transom further forward just to demonstrate the question .... I am asking about the position of the prop along the stern bow line, or centre line ....naturally I will do the needed alterations to the deck once I have fixed my position for the outboard .....

 

[DogCavalry]

A pontoon boat has large directional stability. The further forward the outboard is, the less steering force it can provide, as the line of thrust gets closer to the centre of lateral resistance. But there's an acceptable range. You look to be still within it.

 

[JohnMarc]

A pontoon boat has large directional stability. The further forward the outboard is, the less steering force it can provide, as the line of thrust gets closer to the centre of lateral resistance. But there's an acceptable range. You look to be still within it.

Thanks for that, I will do a redraw and post a pic with dimensions if you wouldn't mind casting your knowledgable eyes over.... will the turbulence of the prop not cause an issue possibly cavitation with the wash from the pontoons? Once again apologies for not knowing the correct terms

 

[DogCavalry]

It's fine to not know the terms. This website will die if folks who don't already know the answers don't ask questions.

My engineering sense is quite highly developed but there are plenty of folks on here that know far more than I do, and often give me good advice.

Unfortunately occasionally someone with a lot of knowledge will be unkind to folks who know less than they do. I hope that never happens to you.

 

[JohnMarc]

It's fine to not know the terms. This website will die if folks who don't already know the answers don't ask questions.

My engineering sense is quite highly developed but there are plenty of folks on here that know far more than I do, and often give me good advice.

Unfortunately occasionally someone with a lot of knowledge will be unkind to folks who know less than they do. I hope that never happens to you.

Appreciate the heads up.....I do try to not take what some folks say personally but rather see it as a reflection of who they are......after all kindness is a gift everyone can afford

 

[bajansailor]

How long will your pontoon boat be JM?
I am guessing it will be about 8' wide, re being road trailerable?
And what sort of speed is it required to achieve?
Just a thought, if you want to go relatively fast, and / or have good manoeuverability, you could perhaps consider having an outboard on the transom of each hull? And the deck could be extended / cantilevered further aft between them?

 

[JohnMarc]

How long will your pontoon boat be JM?
I am guessing it will be about 8' wide, re being road trailerable?
And what sort of speed is it required to achieve?
Just a thought, if you want to go relatively fast, and / or have good manoeuverability, you could perhaps consider having an outboard on the transom of each hull? And the deck could be extended / cantilevered further aft between them?

It is 21' x 8'...... I am constrained by the allowable length in our small boat harbour where I will be mooring it. The idea I had was to bring the motor "into" the deck space between the pontoons as far as the physics would allow to give me the swim steps from the back of the pontoon and more space for my kitchen. If I place the outboard at the end of the pontoons I would have to shorten both pontoons considerably as the dimensions are taken including the outboard.
Speed is not a criteria, we are fortunate enough to live on an island in a beautiful lagoon so it is effectively a viewing/eating deck so I would imagine 10-12 knots would suffice, I am trying to understand the impact it would have of manoeuvrability, I am even considering installing an electric trolling motor to get me out of the little harbour. Hence my dilemma of looking for a way to "calculate" the impact the length of the pontoons versus the position of the prop, if there is such a thing of course .

 

[Mr Efficiency]

The idea I had was to bring the motor "into" the deck space between the pontoons as far as the physics would allow to give me the swim steps from the back of the pontoon and more space for my kitchen. If I place the outboard at the end of the pontoons I would have to shorten both pontoons considerably as the dimensions are taken including the outboard.

I don't see any problem at all with bringing it in a couple of feet, so it doesn't protrude, the boat will still steer.

 

[JohnMarc]

I don't see any problem at all with bringing it in a couple of feet, so it doesn't protrude, the boat will still steer.

Many thanks for that

 

[ondarvr]

Moving the motor forward will create some minor differences that would be trumped by the benefits of doing it.

 

[JohnMarc]

Moving the motor forward will create some minor differences that would be trumped by the benefits of doing it.

Yes it is the "minor" differences I am trying to have an understanding of.....i.e. am I forever handicapping myself for example with speed and manoeuvrability .... Is there a calculation that can be done which will give me an indication of the differences due to the positioning of the outboard.
Excuse my ignorance but I am if nothing else keen to understand this issue. I do of course have the option of shortening the overall length to extend the motor further back from the stern if the benefits were worth it.......but I am loath to give up the extra space ....i.e is the BBQ more important than the compromises? I cannot answer that question without knowing the latter part of the formula....

 

[ondarvr]

There will be .06243 noticeable difference in handling.

Don’t over think it, pontoon boats are large platforms to have fun on, handling is about about fifth or sixth on the list of concerns (maybe even further down).

 

[JohnMarc]

There will be .06243 noticeable difference in handling.

Don’t over think it, pontoon boats are large platforms to have fun on, handling is about about fifth or sixth on the list of concerns (maybe even further down).

Mmmmm..... good point....yup guilty ..... now Mycell?, Lycell?, Epoxy?,Poly?......lots to keep the curious mind occupied.... thanks to all for your advice.