Make sure the trailing edge (the widest part) is about centered over the prop for proper performance.<br /><br />To answer your question Robby, becuase every boat is different. On some boats they work great and on others they can make things worse.
same reason they sell 26 ft boats with out trim tabs and bilge pumps that are so small they can barely keep up with a good rain shower <br /><br /> i like my doel fin<br /><br />and after hearing how evil fins are all winter i started my yearly week at lake geroge with out it and tested my speeds and performance with and with out it<br /><br />well with the 19 in prop i went 41 mph without it and 41 with it and had a much better ride with it pretty good speed for a 3.0 on a 20 ft boat and i was reving past 4800 i can get 43 mph with a 21 but i dont run the boat empty very often<br /><br />when i put my bro and are 4 now full size 17 to 20 year olds in the boat to go into town it wont plane with everybody sitting in there seats<br /><br /><br />which pssed everybody off put it back on even put 2 200 pound guys in the back seat what you know it has no problem getting on plane <br /><br />with 6 guys on the boat and the lighest one at 170 it still did 39.5 MPH <br /><br />i realy like wasteing money like this <br /><br />now looking at some of the other COPYS of the orginal doel fin i can see how they might cause problems because there way to BIG and extend much to far past the rear of the cavation plate<br /><br /><br />tommays
<br /><br />Chris.............
. your boat is much bigger than mine for sure. but i had the stingray type fins on my 165 ever since i can remember. well, i'm with achris here. broke off my ventilation plate. now i got a piece of alum. screwed onto my vent plate, starboard side, cause i aint found a aluminum welder that's hip.. i don't see, at this point much differce without them to with them as there gone on my boat now. on plane, i do think my ride has bow up a bit more without them with full down trim. comeing to plane. not much diff. low speed plane not much difference. also when i took the fins off, i find that my vent plate is broke on the port side too, due to the fins. someone was able to aluminim weld that side good enough. <br />so i say tabs too. if i do anything for what i got it will be tabs. perhaps i don't need anything, and yer boat being so big will need some extra help to get on plane, i don't know. but like bondo says too, i don't like the idea of the extra stress that most likely is on the drive idea either. let alone breaking off vent plate pieces. <br />one last note for my opinion on this. i just keep thinking about how much extra stress is put on the vent plate, and the drive in general when i'm at wot and the boat is at a 45* angle, just below plane, and thems fins are down there trying to rip my stuff up just to lift the back end up so i can plane faster. trim tabs just seem like a better idea to me.I think you may be on to something tommays. Doel Fins, properly installed on my pre-Alphas are no where near the end of the cavitation plates. The trailing edge is supposed to be centered over the prop which is a good 3" or so from the end of the plate. Stingrays and the other brands of hydrofoils are very different. Doel Fins are 2 independent pieces rather than one piece like a stingray. And as you say, at their widest, they don't overhang the plate all that much. They are also not a purely flat plate but are contoured. So, there is a difference in hydrofoils.
My experience on my 18 Ft Rinker with a 3.0 with just the opposite with the Stingray. I didn't like it so I took it off. It planed about the same, cut my top speed a touch, and really altered my steering response in hard turns at speed...especially in boat wakes...so I felt it may be dangerous.<br /><br />Every boat responds differently. Might be a good crutch for an older boat without a good hull design, but for a well designed boat that's properly setup it should be unnecessary and generally will negatively effect handling. Small, underpowered or overweight boats get the most noticable difference and planing improvements.<br /><br />That said...I've never tried a Doel fin, and they are a distinctly different design from the Stingray.<br /><br />So...try the fin and see how you like it. If you don't like it, take it off and Ebay it. If you do, keep it. But my guess is that I don't think you'll find it to be a good product for a boat your size.
