25 Bertram Prop Pocket Design & Construction

Designing a prop pocket was very difficult for me the first time on the 22' SeaRay. The difficultly and stress came from not knowing what affect this would have on the hull's performance. Would the prop cavitate? Would the boat handle weird, and or be unsafe? Is the rudder too close to the propeller? Will the boat respond to the rudder? Will the pocket tear out if I hit something and sink the boat 50 mile offshore? It was basically a "I think this will work" type design trying to stay conservative and on the side of caution.

These are just a few of the issues you can be faced with if you design your own pocket. For me it was the stress of ruining a perfectly good hull. Once the boat hit the water and ran great all fears went away except the one about hitting something. Once I had grounded the boat a couple of times, hit a submerged pipeline and screwed up a couple of props....without any pocket damage that one went away also.

The glass in the pocket is around 1 1/2" thick. A prop pocket is no place to go cheap on materials. A properly designed and installed pocket can actually strengthen the hull. I wouldn't say that the SeaRay's pocket is any stronger than the original design of the deep vee but I will say that's it's more than adequate. Having one design and build behind you creates lot's of confidence that you can repeat the process with similar results.

Picture on the right is the finished pocket insert being trail fit into the 25 Bertram hull. A near perfect fit was not anticipated but was a welcome relief. At this point the pocket is ready to begin the installation.

Here are three of the considerations that I used when designing the pocket for the 25 Bertram.

FIRST CONSIDERATION; keep the pocket as shallow as you can. To avoid drastically changing the performance of any hull try to alter the hull as little as possible. The main reason to do this in the first place is not to alter the performance, but rather to gain the ability to reconfigure a small (21-26 ft) deep vee boat into an in-line inboard. This design is NOT intended to create a super-shallow draft boat with the prop completed recessed into the hull. With that noted the pocket DOES have a very positive effect on reducing the boat's draft. Estimated draft on the 25 Bertram is around 28". The 7" or so that the prop is raised may not sound like that much but where I use the boat the difference between 28" and 35" has an impact on the areas that I can run. These draft figures represent the boat sitting idle, non-planning mode.

This design facilitates a compact, in board installation, remember that only 6"-7" of prop pocket translates into a 2' difference in engine placement, critical for shorter boats.  A good rule of thumb to follow is to design the pocket in such a manner that the highest location of the propeller's functional blade area is only about 20%-30% above the original keel line of the boat. I find this has worked well and the pocket will have no problem feeding water to the top of the prop. Basically the prop can stay completely submerged under these conditions, at these relatively low speeds speeds, say under 30 kts. When you take into consideration the planning attitude or angle of the boat you can see that the prop will have no problem finding good water to run in. The SeaRay actually ran around 38 kts on the first day out without any signs of cavitation in the pocket at all...and with a small block Chevy. The efficiency of this drive train in this hull has proven to be in the 80%-85% range, or 15%-20% slippage.  For an inboard boat that's good, 90%+ would be considered great. 100% is unobtainable, there is no such thing as a prop that doesn't slip to some degree. More on efficiency calculations later.

The pictures below are a professionally designed and installed prop pocket into a 25 Grady White. I found Mike Towey on thehulltruth.com site and he was happy to send me some shots of the installation. Oddly enough this pocket is nearly identical to the one I designed in my attic for the Bertram? I believe this pocket provides more tip clearance than mine, but it may have been designed for multiple applications. It always makes you feel a little better when someone else is doing the same thing that your are planning. It may not be a Bertram but this boat is cool and Grady's are built like tanks.

Apparently the man responsible for the design is named Jim Wynne. I'm sure an internet search would locate him and his services. Image above are his design. I believe this pocket is actually shorter than my design and would allow the engine to be located even farther aft. Not having the hydrodynamic experience that he obviously does I think I leave mine just like it is.

Take a look at the pocket on the SeaRay after 4 years of use. Note that it is very shallow and basically designed to bring the drive train into alignment with the engine in the required location. This pocket has little or no impact on hull performance as originally designed. Pictured is an 18" diameter prop running on a 1 3/8" shaft, and held in place by a custom fabricated SS strut. The pocket pictured below has a total length from the transom to the stuffing box flange of 51". The SeaRay hull is a Raymond C Hunt designed 20^o modified deep V, very similar to the early Bertram's. For those who would said that "this won't work in a boat designed for a stern drive"....here it is and it works just fine.

SECOND CONSIDERATION; Keep the pocket as short as you feel comfortable. Remember that the goal is to be capable of pacing the engine as far aft as is possible. Keeping the pocket short will require a decision that will affect the rest of the build, the shaft angle. If you start out trying to find "the official inboard shaft angle" you will find learn quickly that this does not exist. You can however, design the angle within what I would consider normal practices. Normal practices for inboard boards are anywhere between 9^o and 15^o . The flatter the angle, or lesser the angle, the more desirable. Keep in mind that the flatter the angle, the farther forward the engine needs to be to provide proper oil pan clearance.  Although I have had more than one person tell me that the angle on the shaft within the normal practice range has little to no effect on boat speed or performance.

THIRD CONSIDERATION; How much aft buoyancy do you want to remove? By recessing the hull in the stern of any boat you have basically changed the hull volume, or reduced buoyancy. This essentially has the same affect on a boat as would adding weight into the aft section of the hull. For boats that were originally designed to carry the weight aft in the form of stern drives this reduction in buoyancy is typically a small plus if you are doing a reconfiguration to in-line. Water is real heavy stuff, that's why boat's float. The amount of water a boat displaces equals the boat's weight (just in case you didn't know that)

I don't have the hydrodynamic testing equipment or knowledge to know what is going on in the pocket with the boat on plane, but the exiting wake from the pocket is smooth and appears non-turbulent. I based the success of this design's flow pattern on the bottom paint....if the paint stays on, It's good. If the paint is eaten away, it's bad. Tearing off bottom paint would indicate that excessive cavitation is going on in the pocket and would eventually began eroding the hull itself. The paint stays on. Well at least as good as anywhere else on the boat's bottom!

THE DOWN SIDES

One...designing and building a prop pocket does produce one nasty little by-product, a dry shaft log. I found out the hard way that the stuffing box receives almost no water when the boat is on plane. No problem idling around, but up and running and the boat simply runs away from the water at the recessed shaft log. This is really an easy problem to solve with either a drip-less packing system or a water injected packing gland. Both require raw water is fed from the engine to the stuffing box. Like I said...no big deal IF you know in advance!

This affect of the dry box is indicating that the water flow from the vee into the pocketed area is missing something... water, and therefore void of air also. A vacuum has formed at the forward end of the pocket. I have noticed that some current thought is out there about ventilating prop pockets. There is a company today selling a pre-fabricated pocket that ventilates with the exhaust gases and utilizes a surface piercing propeller...cool. They claim some very impressive speed numbers. I not going to be trying this on the Bertram, but I may try ventilating the pocket with a thru hull fitting and a hose. I'm a little curious if this would have any affect on the performance? The vacuum may actually help and be required to pull the water up and into the prop as it passes into the pocketed area? Beyond my level.

Two... The compact drive line lay out means that the rudder will extend beyond the transom...a little, based on the size rudder that will be used. I have found this not to affect the boat's handling. Single inboards handle fine idling around or on plane...and like pigs around a dock. Learn to live with that fact, get accustomed to the boat's handling characteristics...or install a bow thruster. This is common with ALL single inboards...not just this conversion.

LAYING OUT THE POCKET

You could start from scratch and come up with your own pocket design or you can just use the lengths, angles and dimensions that I have found to work well in boats of this size. Seems a lot easier, after all I'm putting all this in print to save you the trouble and stress of figuring it out on your own...or for you to decide that you want no part of it!

Start by building a plywood side profile of the pocket that reflects how the pocket will be shaped. My is 50" long. Remember the goal of not placing the engine PAST the CB (10ft). Do a little math; (Pocket / recess =50")(stuffing box, packing, companion flange = 12") (3208 CAT engine overall length = 60")...totaling 122", or 10ft 2" from the transom to the nose of the engine. The 3208's block, heads and gear are actually only around 45"-50" long and the last 10"-15" is relatively light weight coolant tank and accessories. This places the "face" of the engine block  around 9 ft-9 1/2 ft forward of the transom. That should keep the boat designers out there happy. 

The pocket has a total internal vertical height of 6". Note I said "internal". Don't forget to allow 3/4"-1" for the thickness of the hull at the keel because the pocket will sit inside the hull once installation.  My overall recess will be 7" higher than the original keel line once the pocket is installed. The image below provides the basic profile of the prop pocket.

Take a look at the side profile of the prop pocket. It's good practice to lay out the entire drive line. This also provides a pattern for the strut and the strut location. I used a 1 1/4" wooden dowel rod to represent the shaft. The hull to shaft penetration flange face is designed to accept a 4" diameter standard stuffing box flange for fiberglass boats and set on a 12^o angle to provide a 90^o shaft to hull penetration. The lower edge of the pattern represents the keel line of the boat. Based on the keel line you can see that the prop tips are only around 25% -30% recessed in the pocket. Red lines on the plywood pattern indicate the location of the transverse pattern ribs.

The dimensions are based on the proven design that I used in the SeaRay, no need to re-invent the wheel every time you lay out another drive line. Stay with a profile and spacing scenario that is proven effective. This lay out allows the 18" prop to be removed with out removing the rudder and provides a 2" tip clearance. Set up show is a 12^o shaft angle.

Side note on shaft sizing;  Bigger is not always better. The calculators on boatdiesel.com uses a safety factor of 2. I suggest that anyone attempting to design their own inboard drive train read the following David Pascoe article on shaft alignment. The article also discusses shaft sizing. http://www.yachtsurvey.com/Alignment2.htm  Based on practical considerations and the advice from this article I will be using a 1 1/2" shaft. Although I believe that the short drive line may actually benefit from a 1 1/4" or 1 3/8" shaft. The shorter the shaft, the less it is going to able to flex...and the hull is going to be flexing a fair amount. I'll get more into hull re-enforcement later.

Back to the pocket layout. Transfer the side profile onto transverse ribs and place the ribs into a prefabricated bed that is representative of the hull profile. I used all thread rod  providing a method to adjust the transverse framing structure before placing a skin over the ribs. I'm not going into the details of creating all the curvatures in the profile, my skills with the English language aren't that good. I will state that the arc is based on a 24" diameter circle with a 6 1/2" wide flat area at the top to provide a flat, stable mounting platform for the strut and rudder packing gland. This keeps you from having to fabricate a strut with  curvature in the based. The flat base in the top of the pocket diminishes as it transitions forward, down to around 2" wide at the shaft penetration face.

The amount of buoyancy that this pocket will remove is estimated at 150-200 lbs, or approximately 25 gallons of displacement. This will be verified once the actual pocket is built by flipping it over and filling it with water....just count the gallons going in and multiple by 8lbs per gallon.

The plug is skinned with 1 1/2" x 1/8" plywood strips and covered with fiberglass. The plug is sanded smooth and primer is applied. The pictures of the plug below are sanded and primed, but the top coat of paint is yet to be applied. Once the top coat is on the plug it will be wet sanded and finished with 1200 grit paper.

The plug will require a mold release agent coating prior to laying the actual prop pocket onto the mold....gotta have a way to get it off.

Here is the pocket installation profile that I am using to install the pocket. This diagram makes a little sense of the "lip" surrounding the lower edge of the pocket insert mold. The lip provides a molded in recess to facilitate a tie-in on the exterior of the original hull. The insert becomes a monolithic element of the hull with tie-in's internally and externally....strong stuff.  Although not the correct scale or shape and the interior tie-in will actually encapsulate the entire pocket, but the diagram provides the basic concept of the insert installation. Once the insert is fully encapsulated the pocket will be more than twice the thickness of the original bottom.

The completed pocket insert mold/plug. Still needs a bit of trimming and the application of mold release but it's pretty much ready to go. Initial fabrication of the pocket insert will include 8-10 layers of 20oz cloth saturated with a 1:1 epoxy laminating resin.