angusmacgregor
New member
I intend to install a small outboard forward of the transom on an 18 ft sailboat retaining the tiller. Can anyone offer advice on design requirements or known problems. Grateful for information.
Outboard installation
- Thread starter angusmacgregor
- Start date
G
Guest
Guest
By "forward of the transom" do you mean in a well inside the boat?
If so - must be big enough to take outboard - must be high enough to give decent freeboard and low enough for prop to locate below hull - must be well ventilated to allow engine to "breathe".
If you mean "to one side and forward of the rudder" then - make sure outboard doesn't foul rudder (sound of prop mincing rudder not good) - leave enough space to allow outboard to be "off-set" so that although it is to one side of the boat it can still drive the boat straight ahead when the rudder is fore and aft. Try to ensure that outboard isn't so far outboard and so low down the transom that it is submerged when the boat is at maximum heel.
If it is "portable" and more than 5 - 6hp brace yourself for a hernia lifting it in and out (or on and off).
Best regards
)
Ian D
If so - must be big enough to take outboard - must be high enough to give decent freeboard and low enough for prop to locate below hull - must be well ventilated to allow engine to "breathe".
If you mean "to one side and forward of the rudder" then - make sure outboard doesn't foul rudder (sound of prop mincing rudder not good) - leave enough space to allow outboard to be "off-set" so that although it is to one side of the boat it can still drive the boat straight ahead when the rudder is fore and aft. Try to ensure that outboard isn't so far outboard and so low down the transom that it is submerged when the boat is at maximum heel.
If it is "portable" and more than 5 - 6hp brace yourself for a hernia lifting it in and out (or on and off).
Best regards
)Ian D
angusmacgregor
New member
Thank you Ian for your reply. Yes the outboard is to go down a tube centrally in front of the the transom. A 3hp Yamaha Malta, not a hernia machine. What is best for the cavitation plate, level with the underside of the hull perhaps? Is a problem with exhaust gasses likely? Different when moving? How close to the unbalanced rudder? The tube and the necessary glassfibre work are not a problem and I expect to cope with the thrust by bracing the upper part of the tube. Thanks, work will get underway in September I expect. Angus MacGregor
charles_reed
Active member
The big problem with outboard wells is the engine exhaust fumes coming up inside the well, mainly because the exhaust outlet is above the bottom of the hulls.
This problem recurs in a number of commercial designs.
Try and ensure the exhaust is below the hull and there is as little space as possible between rear of well and engine leg.
This problem recurs in a number of commercial designs.
Try and ensure the exhaust is below the hull and there is as little space as possible between rear of well and engine leg.
G
Guest
Guest
I agree with Charles - try to give the engine as little space as possible to let the exhaust come into the boat.
As a general rule - forward is not a problem it's when you are going astern, sat there waiting for the engine to warm up or sat in a lock with the engine running.
The only problems I can think of that could affect the cavitation aspects is if the shaft wasn't deep enough or if the hull shape prevented water reaching the propellor (e.g. tight up in front of a skeg). Otherwise with such a small engine there should be no problems.
On the other hand the small engine may be a problem. I'm a great believer in fitting "just enough" engines and running them flat out. They seem to last a lot longer than those that only tick over to give the desired HP.
Mind you, I over-cooked this principle one time with a 4 hp Johnson and a Leisure 17 sailing in the Humber. "Flat out" at most times equalled "just stemming the tide". It was disconcerting going full ahead trying to reach South Ferriby lock and watching the shoreline going in the worng direction!!
Before you design and build in an outboard well make sure that it is big enough to take an engine size suitable for the boat and the sailing that you will be doing.
One other thing when designing the engine well - see if you can still retain the features that make outboards so damn good at what they do. Try to keep the steering ability if you can. It makes the boat incredibly agile in close quarter manouvering if you can swng the tiller hard over AND move the engine in the same direction.
Best regards)
Ian D
As a general rule - forward is not a problem it's when you are going astern, sat there waiting for the engine to warm up or sat in a lock with the engine running.
The only problems I can think of that could affect the cavitation aspects is if the shaft wasn't deep enough or if the hull shape prevented water reaching the propellor (e.g. tight up in front of a skeg). Otherwise with such a small engine there should be no problems.
On the other hand the small engine may be a problem. I'm a great believer in fitting "just enough" engines and running them flat out. They seem to last a lot longer than those that only tick over to give the desired HP.
Mind you, I over-cooked this principle one time with a 4 hp Johnson and a Leisure 17 sailing in the Humber. "Flat out" at most times equalled "just stemming the tide". It was disconcerting going full ahead trying to reach South Ferriby lock and watching the shoreline going in the worng direction!!
Before you design and build in an outboard well make sure that it is big enough to take an engine size suitable for the boat and the sailing that you will be doing.
One other thing when designing the engine well - see if you can still retain the features that make outboards so damn good at what they do. Try to keep the steering ability if you can. It makes the boat incredibly agile in close quarter manouvering if you can swng the tiller hard over AND move the engine in the same direction.
Best regards
)Ian D
G
Guest
Guest
On a Snapdragon 23, I changed from transom mounted lifting bracket, to my own built cockpit well.
I cut the hull at centre as far back in the cockpit as possible and made the shape like a V - point forward. This to allow the engine to be pivoted and so steer at low speed. MAKE SURE you resin the raw cut edges well before any water gets near it ! The edges must be sealed. (Don't be surprised about a) the thickness of the hull here, b) eating jigsaw blades ..... I went through 5 just cutting this hole......
I raised the cockpit sole to 1/2" higher than the waterline, by fitting 2x2 softwood around the cockpit seal and then screwing the original sole board down.
The O/B well had a face board of 3/4" Marine ply fitted across between the cockpit seating. It was cut to shape to fit the contours of the hull / seating sides. First a mix of Micro-Ballonns and resin was made and applied all around one side of the joint. This set in 15-20 minutes enough to hold the face board enough to fillet the joint from the other side. Copious amounts were applied in successive layers to build up a profiled fillet. Then chopped strand matt, medium weight was then applied in strips for about 6 layers either side of this joint overlaying by about 25% of each strip end.
Once all was set, I then drilled through rom the inside of the cockpit lockers into the end grain of the face board each side for 3 dowels to be glued through the locker sides into the face board edge each side.
A second shaped piece of 'scrap' ply was then glued and screwed to the top centre edge of the face board to accomodate the o/b holding bolts.
Note that succesive cuts and trials on dry land were made to the face board to get the prop at the same level as when it was on the lifting bracket working position before fitting the second pad piece.
Once all set, screwed etc. I painted it all the same blue as the rest of the cockpit and believe me it was the best thing I ever did to that boat ..... I could slot her into 25ft gaps, manouevre like a top ..... it was incredible the difference having the prop in front of the rudder and also being able to turn the engine about 60 deg. either side ........
Really all I did was think about it logically and make sure that I didn't flood the cockpit / have a weak mounting. Oh Yes I bought 2 plastic wash pipe connectors for domestic use and fitted those through the base of the new board above cockpit sole to drain the cockpit into the well. I used those because I could screw on the blanking caps when necessary.
As to a top cover ..... never bothered, main sheets were OK, engine was below tiller, etc. never needed it. The only problem was when not moving and the engine was running .... exhaust vented into the cockpit, but once moving fell astern OK.
What engine ..... bloody heavy Mercury 7.5 SailDrive ..... nearly pulled me in the harbour a few times on and off that transom bracket !!!!!
Good luck
If any more info needed ....... nluther@solent-life.co.uk
I cut the hull at centre as far back in the cockpit as possible and made the shape like a V - point forward. This to allow the engine to be pivoted and so steer at low speed. MAKE SURE you resin the raw cut edges well before any water gets near it ! The edges must be sealed. (Don't be surprised about a) the thickness of the hull here, b) eating jigsaw blades ..... I went through 5 just cutting this hole......
I raised the cockpit sole to 1/2" higher than the waterline, by fitting 2x2 softwood around the cockpit seal and then screwing the original sole board down.
The O/B well had a face board of 3/4" Marine ply fitted across between the cockpit seating. It was cut to shape to fit the contours of the hull / seating sides. First a mix of Micro-Ballonns and resin was made and applied all around one side of the joint. This set in 15-20 minutes enough to hold the face board enough to fillet the joint from the other side. Copious amounts were applied in successive layers to build up a profiled fillet. Then chopped strand matt, medium weight was then applied in strips for about 6 layers either side of this joint overlaying by about 25% of each strip end.
Once all was set, I then drilled through rom the inside of the cockpit lockers into the end grain of the face board each side for 3 dowels to be glued through the locker sides into the face board edge each side.
A second shaped piece of 'scrap' ply was then glued and screwed to the top centre edge of the face board to accomodate the o/b holding bolts.
Note that succesive cuts and trials on dry land were made to the face board to get the prop at the same level as when it was on the lifting bracket working position before fitting the second pad piece.
Once all set, screwed etc. I painted it all the same blue as the rest of the cockpit and believe me it was the best thing I ever did to that boat ..... I could slot her into 25ft gaps, manouevre like a top ..... it was incredible the difference having the prop in front of the rudder and also being able to turn the engine about 60 deg. either side ........
Really all I did was think about it logically and make sure that I didn't flood the cockpit / have a weak mounting. Oh Yes I bought 2 plastic wash pipe connectors for domestic use and fitted those through the base of the new board above cockpit sole to drain the cockpit into the well. I used those because I could screw on the blanking caps when necessary.
As to a top cover ..... never bothered, main sheets were OK, engine was below tiller, etc. never needed it. The only problem was when not moving and the engine was running .... exhaust vented into the cockpit, but once moving fell astern OK.
What engine ..... bloody heavy Mercury 7.5 SailDrive ..... nearly pulled me in the harbour a few times on and off that transom bracket !!!!!
Good luck
If any more info needed ....... nluther@solent-life.co.uk
G
Guest
Guest
Read my previous reply on this posting .....
Having done this job, I found out what was good and bad .... luckily the bad was minor and far outweighed by the good.
The exhaust problem cannot be cured by making the aft bulkhead close to the leg, in fact unless you can duct /extract the gases, you are stick with them unless actually moving ahead.
By the way ..... a little trick for Mercury owners, and maybe possible for others ..... exhaust exiting the prop boss can cause cavitation on going astern, so a well placed drilled hole in the leg casing to exit the gases before passing to the prop can really help.
Having done this job, I found out what was good and bad .... luckily the bad was minor and far outweighed by the good.
The exhaust problem cannot be cured by making the aft bulkhead close to the leg, in fact unless you can duct /extract the gases, you are stick with them unless actually moving ahead.
By the way ..... a little trick for Mercury owners, and maybe possible for others ..... exhaust exiting the prop boss can cause cavitation on going astern, so a well placed drilled hole in the leg casing to exit the gases before passing to the prop can really help.
