CD 28 & Outboard motors

[Jim Gregory]

I just bought a CD 28 w/o a motor and wanted to know if anyone has experience with an OB on a 28. I think a 25" shaft will reach as low as the inboard prop.

Thx-JIm

[Carl Thunberg]

I think you'll find very few people have attempted this. You're in a very unique position having bought a CD28 without an engine, but you had your reasons, so I won't question them.

Forgive me if you already know this stuff, but there are some very practical limitations to what you're attempting. After installing a motor mount, the engine will be WAY aft, making operation from the cockpit problematic. You'll want to invest in an extended tiller. The boat will probably be heavy in the stern, making it tend to squat in the water. An outboard would get you in and out of your home port (slip, mooring, etc.), but you could find yourself under-powered in an adverse current or in heavy chop with a headwind.

You may also find the boat to be more tender than it would have been if it had an engine down low in the boat. With the removal of that ballast, the boat will heel more readily and you could find yourself with at least one reef in anything over 10 knots. Good luck with this, and please let us know how it works out.

[billc]

the author of atomvoyages.com discusses his use of a small outboard motor to power his 28 pearson triton.

[sealark]

Jim,

Welcome to the board - I believe you will find a wealth of valuable knowledge here.

I know the question has been asked several times - and hopefully my personal insight will help you and maybe others in the future.

Let me start out by saying that my opinions here are based on personal first hand experience. I purchased a CD28 with the motor removed and installed an outboard. Then after a couple years I re-installed a diesel inboard. I sailed her several times both ways.

First - I will speak to performance. As far as sailing - I noticed no real difference between having the inboard or outboard. I disagree with the previous posts. The boat was never "more tender" or heeled more easily. I simply don't agree with the notion that removal of the engine will have any dramatic effect on the ballast. The original motor probably weighed in somewhere around 450 lbs. Most people I know routinely add or remove this much weight in the form of equipment and belongings. Additionally - I never really noticed any real negative effect of having the additional weight that far aft. Most people add davits or other equipment that far outweigh the outboard. If 450lbs of ballast affected the performance that much - it wouldn't be a Cape Dory.

Second - the equipment itself. As with anything - for it to work properly - I feel that it is necessary to use the RIGHT equipment. The bracket must be an adjustable bracket with about 15 inches of travel. The motor I used was a 9.9hp with the pusher prop - you will need at least that much power. It must be a 25 inch shaft. IT MUST BE ELECTRIC START AND REMOTE SHIFT AND THROTTLE. Anything short of this and I don't feel it will be effective. As Carl mentioned - operation from the cockpit is beyond problematic - it is impossible. I mounted my bracket perfectly midship at the stern - not offset to either side. I locked the motor in place and controlled everything from the cockpit. If you want to install self steering at a later point - the bracket will be in the way. Anything short of what I described will be nearly impossible. Also - you will NOT be able to get the motor to tilt all the way out of the water (during storage) while on the bracket - so keep this in mind if you are in salt water.

So - it will work. With the the setup as I described above - she was able to handle rough seas and strong currents. If you are planning on extended travel to the islands or something - I probably wouldn't recommend it. If you are using your boat as a daysailer/weekender or coastal cruising - it could be very effective. If you hate the smell of diesel fuel or other fumes inside the cabin - it could be a very good option.

From a resale value - if you ever plan on selling - having the outboard is worse than having no motor at all.

However - my advice to you is this. If you are thinking about it - don't do it if you believe it is a CHEAPER alternative to installing an inboard - because it probably will not end up that way. If you don't currently have the outboard and bracket I described - the cost to purchase them is probably more than a good used diesel inboard - especially in todays market.

I think you will find that most of the negative comments you receive will be from either people that did not use the proper equipment - or simply hate the idea of an outboard hanging off the transom of such a beautiful boat. And I will be totally honest with you - one of my reasons for reinstalling the inboard was for that very reason - I hated the way the outboard looked.

Congrats on the purchase. Hope this helps. If you have any other questions or want to pick my brain - feel free to send me a pm. I pretty much redid my 28 from bow to stern.

Kevin

[Jim Gregory]

Thx very muh for the reply and the offer to answer future questions. U r definitely knowledgable abt the subject. Will keep u posted. No sure abt existing drive shaft that is still in the boat

[Carl Thunberg]

Nice to see that someone has actually done this. I was really just getting the conversation started, and I'm very glad to hear that there was no discernible difference in sailing performance.

[2tocruise]

While I have no practical experience, I have a couple of thoughts:

1. Might want to consider a side mounted bracket instead of transom. I have seen outboard brackets designed to mount the motor on the stern quarter. I have even seen them in bronze too, although I am not 100% sure from where (maybe Bristol Bronze). Probably originally designed for double enders, but should work just as well, and not look nearly as bad.

2. I would think slow speed maneuvering (already not easy with the full keel) might be further hindered by an outboard mounted all the way aft and fixed on the transom. With the prop located behind the rudder there will be no flow over the rudder at slow speeds, so you would loose any effective steering much sooner than with the prop located forward.

3. Take the Skipjack/Bugeye approach. Restricted to dredging under sail by law, Chesapeake workboats used a seperate Yawl or push boat to power them in and out of port (and the two days a week they could work under power). Keep the mothership engine-less and use a larger engine in the dinghy. The dinghy could either be tied alongside (hip tow or power makeup) or used to push from the transom. This would probably only be feasible in protected waters, although I did once see a racer use this approach to motor against a 6-7kt current that was setting them onto a sandbar (I was anchored at the time, waiting for a tide shift and praying I didn't drag onto the bar as I had a busted transmission).

[Dixon Hemphill]

My CD28 weighs 9000 pounds and I can't imagine motoring with an outboard in "difficult" times without a good reliable diesel engine to keep the boat going where you it want to go UNLESS you have a big OB that you are sure will perform.

[WaywardWind]

With the prop located behind the rudder there will be no flow over the rudder at slow speeds, so you would loose any effective steering much sooner than with the prop located forward.

A piece of physics: --PULLING-- water over a rudder (i.e., outboard behind the rudder when starting to go forward, or inboard prop in front of the rudder when starting to go in reverse) leaves NO usefullness of the rudder.

None.

Water --pushed-- over a rudder (ob going backwards, ib going forward) works and gives rudder control. Water pulled gives no control at all (physcially it can't) until the boat is actually moving.

In physics, it is called "The Underwater Lawn Sprinkler" problem, and it made a future Nobel laureate (Richard Feynman) famous as a young man about 70 years ago. MIT has a DIY display set up one can walk up to and play with to one's hearts content showing just that.

With no boat speed, "water pushed over rudder = control" while "water pulled over rudder = no control".

Tug boats have control when backing up because they have a separate set of rudders forward of the prop for use when backing up.

[drb9]

With the prop located behind the rudder there will be no flow over the rudder at slow speeds, so you would loose any effective steering much sooner than with the prop located forward.

A piece of physics: --PULLING-- water over a rudder (i.e., outboard behind the rudder when starting to go forward, or inboard prop in front of the rudder when starting to go in reverse) leaves NO usefullness of the rudder.

None.

Water --pushed-- over a rudder (ob going backwards, ib going forward) works and gives rudder control. Water pulled gives no control at all (physcially it can't) until the boat is actually moving.

In physics, it is called "The Underwater Lawn Sprinkler" problem, and it made a future Nobel laureate (Richard Feynman) famous as a young man about 70 years ago. MIT has a DIY display set up one can walk up to and play with to one's hearts content showing just that.

With no boat speed, "water pushed over rudder = control" while "water pulled over rudder = no control".

Tug boats have control when backing up because they have a separate set of rudders forward of the prop for use when backing up.

On my CD26, where the O/B is aft of the rudder, I still have control. Wouldn't your conclusion suggest that boats with an outboard, all of which are behind the rudder, have no control? What gives me maneuverability is speed.

[WaywardWind]

On my CD26, where the O/B is aft of the rudder, I still have control. Wouldn't your conclusion suggest that boats with an outboard, all of which are behind the rudder, have no control? What gives me maneuverability is speed.

I'm sorry, let me be a little bit more clear. With NO BOAT SPEED, there is no rudder control by --pulling-- water over the rudder. Once the boat is moving, the rudder becomes effective.

With NO BOAT SPEED, --pushing-- water over the rudder gives control from the first instant.

http://web.mit.edu/Edgerton/www/FeynmanSprinkler.html

[drysuit2]

Why are we getting so technical about all this?
It’s all trade offs. People may not notice the changes in trim and steerage. But they are there.
Place an outboard on the stern of a boat designed for an inboard: and of course you lose righting weight down low where you want it.
So you have to compensate for that by adding ballast down low and forward. [it could be as simple as keeping your water tanks topped off].
Place an outboard on your stern: and of course it upsets the boats fore and aft trim. So you need to compensate for that [same as above].
With any power: you need to be moving in order to steer. That is change direction.
Inboards are efficient going forward, and add ballast down low where you want it. Buy they suffer from prop walk and perform poorly in reverse. So you have to compensate for that.
Outboards, besides adding weight to the stern, are not as efficient as inboards in forward. But you can compensate for that by turning your actual motor, thus changing the direction of your prop. Which actually makes them easier to steer in reverse.
And lastly my favorite thing about outboards… when you are having engine trouble, you simply remove the out board, leave it with the mechanic, and go sailing.
Which is what we should all be doing about now.

[Neil Gordon]

With any power: you need to be moving in order to steer. That is change direction...

Inboards are efficient going forward, and add ballast down low where you want it. Buy they suffer from prop walk and perform poorly in reverse.

Technically speaking, water must move over the rudder to change direction but the boat can be standing still. A good blast of "forward" can kick the stern quite a bit.

Help me with the physics of prop walk. Why would an inboard "walk" and an outboard not?

[Duncan]

...Help me with the physics of prop walk. Why would an inboard "walk" and an outboard not?

According to Kiwi Props, that would be because the outboard is, well, "outboard", i.e. prop wash off the hull is not an issue:

PROP WALK:

Prop Walk - or the tendency of a vessel to move it’s stern sideways when engaging reverse and before it has way on – is not caused directly by a side thrust from the propeller but by the wash off the propeller blowing back over the hull and in effect creating a lifting effect to one side from the asymmetric flow from the propeller over the hull.

All propellers have a “ race “ that exits from the rear of the propeller in whatever direction it is operating. This is then deflected by the rotation of the propeller transferring energy into this race away from the centerline of the shaft.

So – the main determinant of “ Prop Walk “ ( in fact more correctly termed Hull Walk ) will be shaft angle – the steeper – the more it blows back over the hull, the shape of the hull, the position of the propeller relative to the hull and of course the volume of water and speed at which it flows back over the hull.

In addition some shafts are offset slightly so they can be pulled easily without removing the rudder and this can of course make matters worse – or better – depending upon which side the shaft exits.

Reflect a moment on this analysis – if this wasn’t the case and prop walk was coming directly from a side thrust off the propeller – the stern would swing just as violently in the other directions when engaging Ahead. Yet this does not happen.

Any extra “ Prop Walk “ in reverse from changing to a Kiwiprop is simply a function of the greater reverse thrust and resulting greater flow over the hull delivered from the three blades which lock into a fixed position in reverse. Typically this will be much more pronounced vs a folding propeller which typically has poor reverse thrust – and this little flow over the hull.

Saildrives have virtually no “ Prop Walk “ as the shaft is horizontal and this minimises flows across the hull. This again shows the impact is from the hull shape and shaft angle – not the propeller itself.

Thus there is nothing from a propeller design perspective that can be done to eliminate or even reduce “ Prop Walk “ – it is a particular vessel design issue...

[WaywardWind]

Technically speaking, water must move over the rudder to change direction but the boat can be standing still. A good blast of "forward" can kick the stern quite a bit.

Again, subject to water being pushed over the rudder rather than pulled.

Help me with the physics of prop walk. Why would an inboard "walk" and an outboard not?

Because the shaft of inboard prop is normally not parallel to the motion of the boat, i.e., the prop is "tilted" relative to the direction is it going. This means that the blade going down in the water has a greater Angle of Attack, meaning "more bite", while the blade coming up has less AoA, "less bite". This gives greater thrust to the side of the prop going down compared to the side coming up.

~~~~Sorry, Duncan, that is not true at all. "Asymetrical thrust" is common on taildragger type airplanes with single, high-horsepower engines. Pilots had to be TAUGHT to NOT apply full throttle to say, a WWII P-51 when in a low speed, high Angle-of-Attack position during an aborted landing BECAUSE the "P-Torque" (Propellor Torque) as it was erroneously called would turn the plane left into the ground.