Keel well trap door

Jaffar

New member
I own a 10-year-old Open 500, a small 5 m, 300 kg, sportboat with a 90 kg rotating keel. In the up position the keel is inside a well  and in the down position a trap door slides into position to close the large opening. I had noticed my boat is a tad slower than other Open 500 in class races. When I dived to clean my keel (task that is impossible to do when the boat is up on its trailer) I noticed that my trap door was slightly open (about 15 mm) in its aft part. Here is the view on land.

20170713_091522.jpg


I live near the plant of Hobie Cat Europe, the new builder of the boat and ask them to fix it. They told me that the trap door has a permanent bent and should be changed. I okayed that having basically no choice. But the technician who issued the report also wrote that "the turbulences consequent to this are not the main reason for the slowing down of the boat as its antifouling has a granular aspect and instead should be the main reason".

It is true that all other class boat I race against are dry sailed but my antifouling is VC 17m Extra. I must also add that I did not clean thoroughly my hull before bringing it to the plant.

My question to the forum is this :

I do not think that this unwanted gap at the end of my keel well is minor to the quality of the finish of my hull.

Do you agree?

PS: more details in French at Fermeture du puits de quille

 
Last edited by a moderator:

fastyacht

Super Anarchist
12,928
2,592
The thing about drag is it is additive. That is causing drag. You can't get that back. The antifouling is a red herring. Yes of course antifouling is slower. But that is not the point. This is also slowwer.

 

Timmys_Trick_Turkey

Super Anarchist
1,604
2
the venturi is designed to empty the water out of the case, but as the sealing plate is bent, you would have massive turbulence inside the case, exacerbated by the venturi. As the water inside the case isnt static, the amount of drag that the open slot and venturie offers is magnified.  On other boats, once the case is empty of water, the venturi sucks air, and doesnt have much drag. As your venturi is constantly sucking water through it (because of the bent filler plate), it presents a substantial drag.   

 

Rantifarian

Rantifarian
the venturi is designed to empty the water out of the case, but as the sealing plate is bent, you would have massive turbulence inside the case, exacerbated by the venturi. As the water inside the case isnt static, the amount of drag that the open slot and venturie offers is magnified.  On other boats, once the case is empty of water, the venturi sucks air, and doesnt have much drag. As your venturi is constantly sucking water through it (because of the bent filler plate), it presents a substantial drag.   
I've wondered in the past whether the air getting sucked down the venturi on centerline increases the chance of the rudder losing grip. The i420s used to suck audibly through the centre cases, and the rudder would let go without warning on a 2 sail reach in big breeze. Was it the venturi, or was it the shite, innefficient foil shape the class required?

 

Jaffar

New member
the venturi is designed to empty the water out of the case, but as the sealing plate is bent, you would have massive turbulence inside the case, exacerbated by the venturi. As the water inside the case isnt static, the amount of drag that the open slot and venturie offers is magnified.  On other boats, once the case is empty of water, the venturi sucks air, and doesnt have much drag. As your venturi is constantly sucking water through it (because of the bent filler plate), it presents a substantial drag.   
Thanks, that's what I think is happening. I also think that the venturi opening is boo big and a smaller one would do a better job as it would have lower drag once the case is empty of water. 

 

Timmys_Trick_Turkey

Super Anarchist
1,604
2
you should be able to replace the venturi with a small piece of half inch plastic hose or copper pipe angled 45 degrees backwards so it doesnt catch seaweed and sucks better when the boat is going forward. Reduce the amount of water in the case by shaping and siliconing styrephone blocks in there. in dinghies with drop centreplates, it is not uncommon to recess rubber flaps around the centrecase slot or silicon 2 peeces of shirtbox lid type plastic over the centrecase slot, which overlaps each other and seals the slot when the centreplate is lifted.  it meant you had to be very careful to not catch weed on it, and be careful when putting the dinghy on the trailer because the rollers would rip it off.  A well fitting seal, low drag venturi, and minimal water inside the case is low drag and makes the boat lighter, which equates to faster. You might want to switch to a hard racing antifoul also, applied with the correct roller, in the direction of the water flow, which means periodic sanding, but it is faster than ordinary thick antifoul slapped on with a paint roller.

 
Last edited by a moderator:

Jaffar

New member
Thanks Timmys, I was also thinking of replacing that large venturi plate but it may take the boat out of its class. I have already fixed the seal between the sealing plate and the side lips. The gap on the picture no longer is. As for the antifouling it is already VC 17 Extra, as hard as it could be, I think.

 
Top