Swim Platform Mod

A forum for discussing boat or trailer repairs or modifications that you have made or are considering.

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Crikey
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Swim Platform Mod

Post by Crikey » Tue Jul 12, 2016 6:26 pm

Some time ago I took note of several members swim platform additions. This resulted in an earlier posting I made following a destroyed mast and some musing about what to do with the left over hardware. http://www.macgregorsailors.com/forum/v ... =8&t=23474
http://s1121.photobucket.com/user/rottn ... 1.jpg.html
The following began with cutting and welding three mast sections along with the incorporation of 30 bolt-rope sliders for the tarp last fall. It wasn't until this spring that I began to buy the hinge and other necessary hardware as well as ,yet, drilling more holes through my transom.
Image
http://i1121.photobucket.com/albums/l52 ... wsxelf.jpg
I made a cut-out template for the tarpaulin I wanted made and let the outfitter decide the stitching and reinforcing regimen. The material is high tech truck tarpaulin (cheaper than my first choice - sunbrella) which won't stretch and will take a large load. This was secured with 120lb uv proof ties (X30).
Image
http://i1121.photobucket.com/albums/l52 ... mup6pn.jpg
http://i1121.photobucket.com/albums/l52 ... 7uass9.jpg
The act of finally lowering the frame adds the right amount of tension to the material though the photo doesn't have the inner clamps attached for holding the working shape. I'd designed the outboard cutout to allow stowing clearance for trailering or lowering with the engine already down.
Image
http://i1121.photobucket.com/albums/l52 ... 6uoojq.jpg
http://i1121.photobucket.com/albums/l52 ... ptjtkd.jpg
http://i1121.photobucket.com/albums/l52 ... jedoyn.jpg
The one finalization I haven't arrived at yet concerns the simple pully and rope I added for the interim. I need either to get it swapped with good non-stretch (bouncy) synthetic material or go to a telescopic pole of some kind. I'm leaning towards the latter as long as I can lower the outboard end directly into the water for my Admiral to easily climb into the boat. She's a stroke survivor and the stock ladder is very difficult for her.
I hope some of you find this solution interesting, and yes I have quite a bit of mast left over for the asking. :)

Ross

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seahouse
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Re: Swim Platform Mod

Post by seahouse » Tue Jul 12, 2016 8:58 pm

That's a great solution Ross, I like it; well done!
(I also have to add that with that bowsprit, any speed you are moving forward at is ramming speed!) :wink:

kurz
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Re: Swim Platform Mod

Post by kurz » Wed Jul 13, 2016 7:04 am

Thanks for this woderful mod!!!

One question:
I see that you have an hydrofoil.

Just me too I have one, but didn't tried yet. Will mount soon.
Can you get some advise?
Did you have any dangerous situation? I think Roger tells not to do not to loose stability.

I have a merc 60HP / Prop 14/10.

Thanks

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Crikey
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Re: Swim Platform Mod

Post by Crikey » Wed Jul 13, 2016 8:10 am

kurz wrote:Thanks for this woderful mod!!!

One question:
I see that you have an hydrofoil.

Just me too I have one, but didn't tried yet. Will mount soon.
Can you get some advise?
Did you have any dangerous situation? I think Roger tells not to do not to loose stability.

I have a merc 60HP / Prop 14/10.

Thanks
Kurz, I haven't experienced anything with it for the four years I've been running. I think the issue they (MacGregor) were most concerned with dealt with the wing style of hydrofoil. That's probably dated a little given that many people now put 75 - 90hp engines on their hulls with good results. My style channels the prop flow and also acts as a trim tab to smooth out the forward and aft motion of the boat under power. Planing is definately easier to achieve with my 60hp.
I had to center the outboard tab because the prop-walk is reduced and the factory angle wasn't needed. If you wanted to do some serious slaloming with no ballast and mast up I might think twice, but as most running is straight line everything works very well.
R.

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Crikey
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Re: Swim Platform Mod

Post by Crikey » Tue Aug 09, 2016 7:05 pm

So I've now had a couple of separate uses of the system and have to regard the uphaul pulley attached to the mast arch as a fail because it flexes too much and would probably begin to bend if I pushed it too much. My workaround was running another line from that to the boom pulley attachment bracket on the boom end to keep the arch forward.
What I'm now wondering about is whether a V ended rod could be deployed on the underside of the platform to engage the lower end of the outboard to utilize its hydraulic system for supporting the platform as well as adjusting its position.
Would it be strong enough? :?
My instinct tells me it's pretty robust in order to be adjustable all the way up to WOT.

R.

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Tomfoolery
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Re: Swim Platform Mod

Post by Tomfoolery » Tue Sep 06, 2016 11:22 am

Crikey wrote:What I'm now wondering about is whether a V ended rod could be deployed on the underside of the platform to engage the lower end of the outboard to utilize its hydraulic system for supporting the platform as well as adjusting its position.
Would it be strong enough? :?
My instinct tells me it's pretty robust in order to be adjustable all the way up to WOT.
Your instincts are good, Ross.

My 50 hp OB pushes the boat to about 16 mph (tops). OB output power is measured at the prop shaft, so no other losses need be considered down to that point, and all the power therefore goes into the prop. Not all of it is converted to thrust, but it all goes into the hub as mechanical power.

Assuming a mere half of the actual shaft power turns into thrust, that works out to 585 lb of thrust at the centerline of the prop shaft at WOT and the boat moving at a steady 16 mph. If the efficiency of the prop in turning shaft power into thrust times speed is higher, then the force is higher (linearly, since the boat speed in this exercise is fixed).

That's a horizontal force at the prop shaft thrust bearing. Adding a strut from the platform to the lower leg will put both vertical and horizontal force into the OB leg, with the proportion and magnitude depending on the geometry and applied vertical load at the platform, but I would expect it to be small relative to the design operating force of the prop.

You have a 60 hp, and without knowing the top speed (and assuming the OB is actually delivering 60 hp at top speed) there's no way to guess what the force is, but at the very least it's likely to be higher at some point in the acceleration curve, up to steady-state running.

Even the hydraulic system, which usually has lousy geometry with the engine all the way down, is designed for trimming the angle under load, so whatever the OB lower leg is designed to do, the hydraulics can do, as far as normal running loads go. And it almost certainly has relief valves built into it, in both directions, for hitting underwater obstructions when moving in either direction.

You can guestimate the force the prop pushes the boat along with using F = HP x eff x 375 / V where F is force in lb, HP is rated engine output power, eff is efficiency (I used .50 for mine), and V is speed at WOT in mph. You have a larger OB, and a smooth-bottom boat without bottom paint, so I would expect your top speed to be a fair bit higher.

I just don't know what efficiency one can expect from a prop in transforming shaft power to thrust and speed, but I've seen sources claiming 65-70% as typical, with 85% being claimed for highly specialized props not normally used by Joe Boater. Those higher numbers would mean higher force at the prop shaft. I'm just trying to be conservative.

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seahouse
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Re: Swim Platform Mod

Post by seahouse » Tue Sep 06, 2016 12:08 pm

Crikey wrote:So I've now had a couple of separate uses of the system and have to regard the uphaul pulley attached to the mast arch as a fail because it flexes too much and would probably begin to bend if I pushed it too much. My workaround was running another line from that to the boom pulley attachment bracket on the boom end to keep the arch forward.
What I'm now wondering about is whether a V ended rod could be deployed on the underside of the platform to engage the lower end of the outboard to utilize its hydraulic system for supporting the platform as well as adjusting its position.
Would it be strong enough? :?
My instinct tells me it's pretty robust in order to be adjustable all the way up to WOT.

R.
I had a similar thought, using the motor tilt to lift a platform, of course the rudders bring an element of complication to the setup. But it would be easy for you to check this by simply standing on the lower unit and activating the tilt (some motors have a second set of tilt switches on the cowl, or get Gabe to help you). Or just stepping on it and using partial body weight to see what power it has. You won't damage anything because hydraulic relief valves will simply lift. The outboard motor in any boat a solid step to use when climbing into a boat.

It also will take more hydraulic pressure to move the motor (up) than to hold it in one place, so a partially tilted motor might support a certain weight without creeping down, but not have enough power to actively lift that same weight higher. So even if it did not have enough power to lift a platform, a little extra manual help (pull on a rope etc) might be all you need to make this arrangement viable.

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Crikey
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Re: Swim Platform Mod

Post by Crikey » Tue Sep 06, 2016 8:09 pm

Well, I've comitted to a salvaged u-shaped piece of stainless tube with a bow roller from Canadian Tire, that I just drilled out today. I figure , similar to your inputs, that the motor can at least position and hold the platform frame at the desired angle, without necessarily having the umpff to actually raise the loading through any subsequent angle change. I'm keeping my digits crossed that it can do both without me having to add my Admirals weight into the speculation pool.
:D
I need two basic positions, one dipped in the water and one out regardless of any in-between capabilities. I should have the final attachment hardware in a day or two and will put in some new pics once assembled. Also, there was a fail with me stepping on, and breaking two of my 120lb cable ties while at this years MMOR. I'm changing these to stainless ties that I discovered at Home Hardware. I also have to source some anti-skid tape to put on the very slippery wetted mast sections.

R.

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seahouse
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Re: Swim Platform Mod

Post by seahouse » Tue Sep 06, 2016 10:16 pm

Lol! It makes it sound like I wasn’t being too complimentary to the admiral, lol. No, I meant if you couldn’t reach the tilt switch while standing on the motor that she could help by working the switch, not by adding weight to the equation, lol. :D

I think too you will have more than enough hydraulic power.

Looking forward to your pix...

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Calin
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Re: Swim Platform Mod

Post by Calin » Wed Sep 07, 2016 9:20 am

Lovely, good job!

KootsChewt
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Re: Swim Platform Mod

Post by KootsChewt » Wed Jul 05, 2017 7:25 pm

Crikey, I'm wondering how your swim platform worked out...

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