Continued…

Now go to the front of the boat on deck. You are going to make an inlet hole in the deck. You want to measure 12 inches out from the main hull gunwale towards the float. Make your mark and then measure 12 inches back from the leading edge of the wing deck. X marks the spot. Drill a 4-inch hole there. This will open the top of your dorade box. The reason I used a 4-inch hole is because that is the size of deck plate I bought. Buy all the deck plates and hardware you will use before you start the project. There is nothing worse then to drill holes and then find out that the deck plates you were going to use have been discontinued and no longer available. Of course, the ones that will be available will not fit the holes you have drilled. You fasten the deck plate to the deck, using bedding compound and screws. You now install a forward facing horn to gather air. I used the low profile ones because they look better. If you are going to be sailing in rough weather and expect to take water over the bow, you can replace your horns with flat deck plates to seal the holes. The Nico ones I have screw in and are watertight.


One last job and you will be finished with the front end. You have to allow for any spray entering your dorade boxes as well as any rain. You will have to drill a 1-inch hole in the bottom of the wing deck, just in front of the number 3 wing bulkhead. You will have to do this by estimation and measuring your boat. The wing bulkhead runs from the main hull number 3 bulkhead out to the float inner hull side. You want the hole in the lowest part of the dorade box. This will be close to the main hull and close to the number 3-wing bulkhead.

You're done! Now when your boat faces into the wind, the horns and the dorade boxes in the front of the boat will provide positive pressure and air flow into the forward cabin. Your under wing elbows will provide negative pressure and will draw out any air in the cabin. I have had to hang a towel over the inside vents in the dressing room on some occasions because there was too much air coming in. It can easily blow out a match in a good blow.

Now for the aft cabin. After you have installed your elbows and PVC pipes and valves, there are just a few things to do. The Searunner design lends itself to positive air pressure for the aft cabin by the forward facing companionway. If you are using drop boards, make an opening vent in the top drop board. A 3 or 4 inch opening will provide ample air movement in the aft cabin. You might want to use a louver plate or opening circular vent. I also crack my back window 1 inch and leave it like that all winter.

I have watched the airflow in the aft cabin using smoke to trace its path. Some of the air that enters the cabin from the companionway, flows under the table and into the compartment behind the table next to the transom. There it turns direction and flows under the floor towards the front of the boat. Since all the bulkheads under the floors are constructed with large holes for airflow, there is very little restriction. The airflow continues forward until it reaches the area under the cockpit. There is an exit tube here and the air, along with any moisture it has picked up, is removed from the area.

I have found that the cupboards and cutouts in the galley remain mildew free. I do not place any plastic bags or anything that does not breathe up against the hull sides or back of cupboards.

Although my boat is tied up to a dock and not at anchor or on a mooring, the ventilation systems works great. If the wind comes from the back of the boat, fresh air enters the boat through the backward facing vents under the wing deck and exits from the front facing horns or companionway. If the wind happens to be coming from the side of the boat, not much will happen in air movement inside your boat that day. The next day, the wind direction will change and your boat will be, once again, ventilated by the free power of the wind.

Now to complete the whole process, we have to take care of the floats. Since the floats are open from front to back, there is less work to do. All we need is an area for air to enter and an area for it to exit. At the front of the float you will have to do a bit of investigation. Look towards the front of the float from the float hatch. You will have to use a flashlight to see way up there. Is the number two bulkhead made with a ventilation hole? If not, you will have to place you air entry after that bulkhead. If number two bulkhead has a ventilation hole, you can place your air inlet forward of this bulkhead.

I measured back 36 inches from the tang in the front of the float. Make a mark here. I found the centerline of the float deck by measuring from side to side. Now drill a hole here. I drilled a 3-inch hole because of the size of the vent I purchased. I mounted a stainless steel mushroom vent in the bows of the floats. I choose a mushroom vent because I felt there will be lots of water over the float bows so I wanted something easily closed and opened. The mushroom vent screws open or closed and is very easy to use. Just spin it and snug it down and your vent is waterproof. Spin it the other way and you have an entry for air.

At the aft end of the float, I measured 12-inches forward from the tang mounted in the float transom. Make a mark here. Again I found the centerline of the float deck by measuring side to side. Once again, X marks the spot. I drilled a 4-inch hole here and mounted a Nico deck plate. I used a rather tall Nico horn in the aft area of the transom to get as much air movement as possible. The system works very well. The floats always remain dry and sailbags, life jackets, etc. remain mildew free.

A little bit of maintenance advice. I would suggest you find out what deck plates you will install and find out what size hole you will be cutting for them. Go out and buy yourself a good hole saw for that size hole. It makes the world of difference to have good tools to do this job. In addition, you will need a beefy drill for this. It takes a ½ inch drill to spin the hole saw fast enough to do a good job. And of course, a nice, new, sharp hole saw works best. When you pick up the elbows you will use, measure the diameter of them and get a hole saw for these holes as well. One last hole saw to buy is the 1-inch saw for the drain holes in the forward dorade box.

If you cut into the plywood of your boat and leave a hole in the skin, you must seal the edges of the ply against water entering. I have always coated the cut edges of plywood with epoxy and put a piece of 4 or 6 ounce fiberglass cloth around the inside of the hole. After it cures, I file it down and give it another coat of epoxy. Primer first then paint with a good quality paint and you will have years of service without any water penetration.

If you do not have a complete set of plans for your boat, I suggest you contact John Marples and buy a set. I went through two sets of plans while building my boat. They get torn, stuff spilled on, and the drawings fade in the sunlight. John sold me the second set for a very nominal fee.

The above modifications are not official Searunner modifications but my designs for my boat. If you would like to use these ideas, please contact Jim Brown or John Marples regarding any safety or structural issues.

You may send me Email if you have any questions or suggestions of how to improve this ventilation system. Contact me at

bob@trimariner.com