Tod Tuna

[ctroutz]

Any Tod Tuna owners out there? I'm really struggling to get advice on the right prop.Lots of help,thanks (including specialists) but no further forward.Most grateful to make contact with someone who's actually got one of these early GRP boats.

 

[Jcorstorphine]

I think you will need to give what engine and reduction gearbox is fitted and what engine revs you would like to run at. It could be that your engine is not the original. From memory I think they were Ford Watermota engines.

 

[ctroutz]

Tod Tuna 'prop'

I think you will need to give what engine and reduction gearbox is fitted and what engine revs you would like to run at. It could be that your engine is not the original. From memory I think they were Ford Watermota engines.

Hi, It's got a 75 tdi Perkins and a 2:1 gearbox. Maxes at about 4500 s'posed to run most efficiently at about 2500. With a 16x13 prop on test it goes well (and lively) to 6Knts + at 2-2500revs. Next 2000 only produces about another Knot. I'd prob. have room for a 17 poss.18 " prop. but I'm getting conflicting advice over whether to go for a higher or lower pitch. (3 blades). Sounds as though you may have seen the same article as me which suggested you could get over 20 Knts with 2 50 hp Watermotas. I'm not expecting miracles but I would have thought that 12- 14 should be attainable! Help!..........

 

[Jcorstorphine]

I have run a quick calc using Surfbaud program but am not happy with the figures so will load another program on one of my other computers as this one has Win 7 and will not run Prop Calc.

One point I would make is that if you can get up to 4500 RPM you are not over-propped but are you sure the gearbox is not slipping. Do you have access to a Tacho such that you can measure prop shaft speed.

Also are you sure about the reduction ratio, might be 3:1.

Other point is that although the Tuna can get up to planing speeds, the displacement speed of a 21 foot boat is only about 6.75 knots so could be the boat cannot get over the hump.

 

[Jcorstorphine]

Something is not right here. I have run three different programs and they are all giving a prop size less than you have at the moment at around 14.5 x 10 to 14.5 x 12.4. So I think you need to check some details

You say you have a 16 x 13, is this actually stamped on the boss or is it just that you have been told these figures.

Are you sure your engine tacho is calibrated correctly, Have you calibrated it against a mechanical or optical tacho

Are you getting full rpm on the shaft, Depending on the design some gearboxes can slip due to wear on the plates, mechanical boxes use a series of multiplates forced together with cones etc. Some can be adjusted other need a new set of plates

Check your gearbox ratio, if there is no marking on it, mark the shaft and turn the engine over one revolution using a ratchet socket on the crankshaft bolt.

When you are at full power is there any black smoke from the exhaust. If there is no smoke then as said in previous post, you are not over-propped or your gearbox is slipping,

How is the boat sitting on the water, is it down by the stern?

Where is the engine, is it right up at the stern with an outdrive or is it further forward. This may have an impact on the boats ability to get over the hump

With that size of engine and the correct prop, you should get a speed far higher than you quote but only if you can get it over the hump. I can give an example of a Birchwood 25 doing 10 to 11 knots with a 75hp Peugeot. The calcs say you should get at least 12 knots if up out of displacement mode

Is the hull clean with no marine growth?

The prop calculators I used were

Surfbaud on an excel spread sheet

VicProp

Prop Calc from Castle marine (Will not run on Win 7)

 

[ctroutz]

Tod Tuna 'prop'

I have run a quick calc using Surfbaud program but am not happy with the figures so will load another program on one of my other computers as this one has Win 7 and will not run Prop Calc.

One point I would make is that if you can get up to 4500 RPM you are not over-propped but are you sure the gearbox is not slipping. Do you have access to a Tacho such that you can measure prop shaft speed.

Also are you sure about the reduction ratio, might be 3:1.

Other point is that although the Tuna can get up to planing speeds, the displacement speed of a 21 foot boat is only about 6.75 knots so could be the boat cannot get over the hump.

Thanks a lot, it's starting to 'narrow down'. Local prop. man came up with a 16x10 but with reservations.
I checked gearbox ratio with 'Newage' at 2:1. Slip has been suggested and I will get hold of some sort of shaft revolution gauge.
I'm also thinking that the high speeds referred to related to earlier models (praps ply skinned). Mines a heavier Grp Lay-up on a heavy carcass I think, I'm guessing about 1.5 tons and I'm trying to offload as much of the heavy on-board equipment I inherited before I give it another shot.
Look forward to your prop calc. Many thanks, Graham. (Also two quite lumpy bilge keels aft dragging a bit.)

 

[Jcorstorphine]

Ok ,I have run the three programs and have come up with the following.

Surfbaud. 15 x 12 but program looks on the boat as displacement hull and could not get it to accept semi displacement or planning type so gave speed as 6.5 knots

Vic Prop 14.5 x 10.4 and gave speed as 13.83 knots

Prop Calc 14.6 x 13.38 but speed of 18 knots.

So it looks as if you should be able to get higher speed than you have at the moment but you should find these programs through Google and run your own figures.

Now, the gearbox. If it is an old style Newage mechanical box (round type with a 4” x 4” inspection cover on the top and a long gear lever) then it has adjustable toggles which can be set by opening the cover and adjusting the three toggles. (if you PM me with your email address I can send you the full procedure) If it is a Newage PRM box, I cant help so you need to seek help elsewhere.

I don’t think there were any plywood Tod Tuna’s built as it was one of the first GRP production boats to be built.

The Ford Watermota engines were quite light compared to your Perkins and there was also Ford Aquaplane engines which were souped up so that is how they may have got the boats up onto the plane.

If you find that your boat is down by the stern, you may find that you need some ballast up forward so it is worth get some 56 lbs weights but be carefull to spread the weight on some ply boards.

The other thing that may help is to fit some form of trim tabs on the transom.

Regards

jc

 

[ctroutz]

Tod Tuna 'prop'

Ok ,I have run the three programs and have come up with the following.

Surfbaud. 15 x 12 but program looks on the boat as displacement hull and could not get it to accept semi displacement or planning type so gave speed as 6.5 knots

Vic Prop 14.5 x 10.4 and gave speed as 13.83 knots

Prop Calc 14.6 x 13.38 but speed of 18 knots.

So it looks as if you should be able to get higher speed than you have at the moment but you should find these programs through Google and run your own figures.

Now, the gearbox. If it is an old style Newage mechanical box (round type with a 4” x 4” inspection cover on the top and a long gear lever) then it has adjustable toggles which can be set by opening the cover and adjusting the three toggles. (if you PM me with your email address I can send you the full procedure) If it is a Newage PRM box, I cant help so you need to seek help elsewhere.

I don’t think there were any plywood Tod Tuna’s built as it was one of the first GRP production boats to be built.

The Ford Watermota engines were quite light compared to your Perkins and there was also Ford Aquaplane engines which were souped up so that is how they may have got the boats up onto the plane.

If you find that your boat is down by the stern, you may find that you need some ballast up forward so it is worth get some 56 lbs weights but be carefull to spread the weight on some ply boards.

The other thing that may help is to fit some form of trim tabs on the transom.

Regards

jc

Thanks so much for your interest. First I'll try to get my hands on a shaft rev. counter to address that possibility. I'll check, but I think it's a PRM gearbox. Thanks again, Graham.