A trim tab question (tabmanII a good one for you)

[f_inscreenname]

I have a question. I fully understand how tabs work but for this one thing that has me baffled and the only way figure it out is to take off my tabs. So let me see if I can explain it. <br /> I have the 12X9’s Bennett’s on that boat over there (it came with them). It has a Volvo drive with no trim When I am running at a plane-ing speed and the tabs are fully retracted do they still push the bow down? I see how they are installed with a ½”off the angel of the hull so does that mean it will only allow the bow to rise that ½” before they start pushing the bow back down?<br /> The reason I ask this is when I took off my old Osco manifolds and replaced them with a set that only weigh 7lbs each I found the bow doesn’t rise as high. I tried to move the outdrive trim pin up to the last hole (it only has 3 holes and I use the center one normally) but the prop was running to high in the water on a plane and wanted to cavitate in turns. I never had much use for the tabs except when a buddy (weight 250lbs+) brought his two buddies (weight, at least 200lbs+ each) along for a ride but normally it just me at a 155lbs and if the whole family goes add another 200lbs . The boat is was balanced near perfect before so now I am looking for ways to get the bow back up. There is nothing in the front of the boat to move back but me and it is hard to drive back by the transom. <br />

 

[Don S]

Re: A trim tab question (tabmanII a good one for you)

That 1/2" gap is supposed to leave the boat as if it had no tabs at all. The water flow off the hull at planing speed would be about 1/2" above the bottom of the boat 9" back if there were no tabs. Of course all boats are slightly different because of hull design and that gap in the water may be different. Such as if you have a rocker or crown in the hull.<br />Your boat is very sensitive to weight, that is why the change to ligher manifolds made such a big difference.<br />If you are going to haul a lot of weight aft and want the boat to ride level without the use of tabs, you might want to move the trim pin to the hole closest to the transom. That would bring your bow down with the extra weight in the stern.<br />With normal load, move the pin back to center.<br />Now you know why Volvo moved away from the drives with no trim a long time ago.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

Hey Don; <br />So what you are saying, in a perfect world, the total bow rise is only going to equal a ½” at 9” off of the transom (end of the tab). Or is it because when the water passes under the hull and comes off of the transom, the hull is on top of the water and the ½” allows the water to pass under the tab so it wouldn’t cause any downward pressure.<br /> I ask again because I have sat in the back when at speed and I can see the tab having contact with the water. How much down force, I don’t know because I have always had tabs on the boat since I have owned it. The boat never needed them to get on a plain or really for any reason. They are nice to have when the bay get whipped up by weather to keep the bow down at below plaining speeds. The real problem I have now is the bow is to low when running at speed. Short of putting 100lb weights in the back seats when I go out I am not sure what to do. Things got me thinking that by adding 9X12” (of basically flat area to the bottom of the hull) to the end of the boat the center of gravity of the boat would be screwed up. Something along the lines of , the transom is supposed to sink so far in the water and because of the tabs and the “extra hull space” it would keep the transom up higher then it should be.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

 

[Reel Poor]

Re: A trim tab question (tabmanII a good one for you)

f_in....you could have a little work done on the prop to create more bow lift. If you can acquire enough bow lift from prop design you can use the tabs for stern lift.<br /><br />My boat will actually run faster with the tabs down, as that allows me to trim the drive higher which creates more total hull lift, ie more hull out of the water so it goes faster. Not much but a few tenths acording to GPS. <br /><br />The purpose of tabs is to lift the stern, not push the bow down.<br /><br />I have even switched my drives from one side to the other to help create more stern lift. Twin engine set-up. I have no problems with cavitation.

 

[QC]

Re: A trim tab question (tabmanII a good one for you)

If you want speed and bowlift I would try shifting enough weight aft till she will slightly porpoise at WOT with the trim setting out as far as you can stand. Use the tabs to just bring the porpoising under control and for hole shot. Makes a big diff with my boat, but I have trim . . . I know I can leave my trim full out though and still plane at very low speeds with my tabs down. I don't think they are pushing the bow down at all full up if they are installed correctly.<br /><br />Where are your batteries etc. I would start finding ways to shift anything aft. The anchor and chain if you have too.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

The prop idea sounds like a good one to start with. <br />I wish there was something I could move back toward the transom. The only thing in the front is me and a life preserver. As for trim, there is none on a Volvo 270 just 3 settings, like a old outboard, with a big bolt. It is a set and forget thing. I was just looking at some pictures of it sitting on flat water and after I changed the manifolds you can see at the transom that the boat sits a good inch higher then it did before the change. It also acts that way through out the power ban. It is not like it plowing through the water but I do notice the bow lower from take off (no big deal, actually better. gets it on a plain faster at a lower RPM) but all the way up to WOT the bow is lower then it was before I changed manifolds and lost a 135lbs off the motor. The best way I can describe it is, "the boat rides flat". For you Merc guys it would feel like it was time to add some trim to the outdrive to bring the bow up. I have tried the third setting on the drive but it cavitates the prop to much. <br />What has to be doe to a prop to make it lift the bow? I would like to tweak the one I have. Super expensive, 15 ½ X 21, cupped, solid hub ocean racing prop for a Volvo 270. I can get on a plain at 2000rpm’s and WOT is 4700rpm’s

 

[rodbolt]

Re: A trim tab question (tabmanII a good one for you)

f_screename<br /> I know where a decent 290 transome shield and DP-A2 drive is that may go reasonable. I just repowered a late 80's 25 albermarle and personally removed the shield and drive package. would make someone a nice cheap upgrade, corrosion was actually minimal on all the parts as nothing had to be heated for remoaval and the helmet shaft and suspension fork pins are tight.<br /> havent a clues what the package will go for used but its all in a pile in a shop floor and must go soon, the saltwater cooled 5.7 is supposed to run well but I never heard it.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

Hey rodbolt; <br /> Will that bolt up to a Volvo bellhousing (like the 270's style bellhousing)? I have way to much time and money to get rid of my Chrysler 360 (I am still paying on the $2000.00 exhaust manifolds I bought this summer) and when I was rebuilding the boat I had to buy a second hand bellhousing and flywheel to mate the drive to the motor. Cost more then $800.00 and took 6 months to find them (that is how I found iboats) many years ago. I am sure it would cost more now if you could even find them at all. So if you have a way it will work with a Chrysler small block motor (and a Volvo 270 drive) I am more then interested but that is the thing that holds me back every time. If I knew what I know now back then I would have scraped everything and used a Chevy motor and the Merc 888 outdrive I have no use for now in the back of the basement. When I started this “boat project” way back when I was trying to keep the boat as original as possible and the transom was (and still is) solid. Now I wouldn’t mind upgrading the drive to something that has trim even though some of the classic boat societies will flip out and try to hunt me down. I posted on the Donzi board about taking everything out and swapping it for a Chevy and Merc set up. I then got many posts and even emails saying not to do it. Things like “you don’t know how rare of a set up that is” (my wallet sure knows) and “sell the boat to someone who will keep it original and get a new boat for that” or “don’t be stupid, you will be taking away the value of the boat”. So I have come to the conclusion if it means changing the transom its not going to happen but if it will fit the same cut out I would be willing to do it. I guess the Holy Grail of drives for me would be is a Volvo 270 or 280 “T” drive. I know they are the same as what I have now but not sure of the 290.

 

[rodbolt]

Re: A trim tab question (tabmanII a good one for you)

the late 280 290 drives are far superior to what you have now, the DP shines even brighter. <br /> best i remember the transom cut out is the same.<br /> the only gig ya may have is the splines on the front yoke. the ones on the drives I just took off are fine spline and I think yours are coarse. however they make a ujoint kit for that.<br /> but if this goes like the rest it will sit about the hop till it gets tossed. like I say I have no clue what it will go for but most likly it will be cheap.<br /> I dont know the condition other than its clean and came apart easy. I pulled all the old stuff and am going back with a new 5.7 and a DP-s setup.<br /> ill try to post to DonS and we can get his input on the interchage. I know the inside edge of the transom may need to be chamfered to clear the hydrualic lines and the trim sender.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

Making the hole bigger is not no problem. Its just trying to make it smaller.... Lets find out if it can be done and at what cost before I will bother you over stuff like gear ratios, price etc, etc. But I am very interested.

 

[rodbolt]

Re: A trim tab question (tabmanII a good one for you)

gear ratio is 1.95/1<br /> that I remember<br />but best I remember its not really bigger its just some chamfering on the inside edge.

 

[Don S]

Re: A trim tab question (tabmanII a good one for you)

Rodbolt<br />I'll do some looking for the template numbers for the cutouts, the hole will be bigger for the 290 style. Does your DP-A2 have the SS bracket around the trim cylinders and also have the lockdown mechanism, or just the trim cylinders and no lockdown like the newer DP-E?<br />It's also going to take an exhaust Y for that transome shield, his won't fit at all.

 

[rodbolt]

Re: A trim tab question (tabmanII a good one for you)

ill have to look next week. but I belive it has the lock down. it does have the round late style cylinders not the square aluminium ones. I think it was repowered about 93 or so last time. <br /> if it would work I think it would wake up that old formula.<br />but it does not have the "elephant ears" type system.

 

[Don S]

Re: A trim tab question (tabmanII a good one for you)

Just to verify, you say it does NOT have item #16 in this drawing.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

“gear ratio is 1.95/1”<br />For a V-8? I run about 330hp through my motor now. I am thinking that ratio may be to high. I thought mine was 1.50:1 or 1.61:1 now. Is there a way to change it??<br /><br />“It's also going to take an exhaust Y for that transom shield, his won't fit at all.”<br />Thru hull exhaust. No need to worry about that. Also no need for water pick up. My pick up is in the bottom of the hull by a raw water pump on the front of the motor. <br />

<br /><br />"if it would work I think it would wake up that old formula"<br /> You didn't call my boat a Formula did you?

 

[Don S]

Re: A trim tab question (tabmanII a good one for you)

Well, I did some checking and the hole will be larger for the DP transom shield and it will fit.<br />You will have to be careful of where the center of the X dimension will be on the DP transom shield when making the hole, but that's not a big deal. I did the math already between the old and new shields and the crankshaft centerline (X dimension) will be about a 1/2" higher than the present setup, so you will have to raise the front of the engine an equal amount. I looked at your site and seen the exhuast setup after my post, so a blockoff plate will be necessary. That also is just a minor part.<br />The 1.95:1 ratio is what you want with a DP, not the 1.61 or 1.50 from the single prop drives..... Trust me on that.<br />Trim wiring can be done fairly easily depending on what rodbolt has from his repower if anything, still not a biggy.<br />Here is a .pdf for the transom shield template that is ok for reading, but you will need a real fullsized one to actually do the cutting and drilling.<br /> Transome cutout template <br /><br />Like rodbolt mentioned, this will bring your Nova to life, and your engine isn't big enough to hurt the drive.

 

[f_inscreenname]

Re: A trim tab question (tabmanII a good one for you)

So the reason for the higher ratio is because the drive has two props? That would make sense. Wow a DP sticking out the transom would flip people out on my old boat. <br /> Hey rodbolt. Now that we have done all this in a public forum I am sure people will be coming out of the wood work for the drive. I hope you will remember me.

 

[rodbolt]

Re: A trim tab question (tabmanII a good one for you)

donS <br /> I cant remember if it has the above item 16 or not. but it has the later style fine splines on the ujoint. and does not have the older elephant ear looking trim cyl cover. it has power steering and ex Y and everything, the guy was fishing offshore with it last week he just wanted a repower. ill try to have a look monday<br /> as far as HP goes I have seen many of those drives behind 7.4L volvos.<br />it will be a lot of manual labor to swap but the addition of the DP and power trim will make a difference in hull speed.<br /> about the only thing I cut removing the package was the ex riser to Y pipe hose.

 

[Don S]

Re: A trim tab question (tabmanII a good one for you)

Rodbolt, <br />Now I THINK I understand what you mean about elephant ears. I thought you meant item #16.<br />You mean like the AQ280DP-PT picture in the drive chart link below. You have the DP-A2, so the transom cutout will be the same as the one I linked to earlier.<br /> Volvo Drive Chart <br /><br />As far as the ujoint yoke goes, if he now has the course splines and the drive you have has ujoints with C shaped keepers on the inside of the yoke holding the bearing cap on and not the outside keepers of the newer style, he can just change the Yoke ---- No Problem.<br /><br />As far as the trim goes, I have diagrams for the OEM system if he wants to go crazy with it, or he can add a momentary toggle switch on the helm and be careful with triming to high. OEM mess is Big Bucks with gauge, relays, wierd wiring harnesses, etc, etc, etc. I just have a momentary toggle switch on mine <br /><br />He will be using his present exhaust system, and will only need to get a blockoff plate.<br />No exhaust through the drive also means less heat in the drive ------ and that's a good thing.<br /><br />I think ya better start boxing it up