Aerating prop

[Ian GardnerParticipant @iangardner62867]

Hi all,

I have found that my Sportsman’s prop is aerating (brief increase in revs) when making a turn. It only happens on one turn and has only been happening since I increased the voltage to 14.4v  from 12v (17+ volts when fully charged according to my charger). I’m very happy with the performance but wonder whether more spray rails under the hull would have helped. It’s too late for this model but perhaps for future reference. There is a picture of this boat in the album and the last one is with the increased voltage. Any ideas?

Thanks,

Ian

[Ian GardnerParticipant @iangardner62867]

Prop aerating on a turn.

[neil hpParticipant @neilhp]

i presume, ian that it is increasing speed when it is turning in the direction of the throw of the prop.

i.e. if it is a right handed prop, turning clockwise when going ahead, it increases speed on a starboard turn.

as the hull leans into the starboard turn, the port side of the boat will rise slightly, and if fast enough, the tips of the prop will start to cavitate [thrash water and air for grip].

this is why fast electrics and full size prototypes are fitted with trim tabs to keep the stern and hull from digging in on such a fast turn, thus running on an even keel, and in fact giving the prop even bite in the water at all times.

try  fitting trim tabs to the stern.

the increase in speed is because there is less load on the prop and motor.

neil.

Edited By neil howard-pritchard on 23/04/2010 19:21:37

[Ian GardnerParticipant @iangardner62867]

Hi Neil,

Thanks for the suggestion. I understood what was happening, just didn’t know how to fix it. I’ll try a trim tab on that side ,see if I can keep the hull more upright in a turn- obvious now you have pointed it out!

Cheers,

Ian

 

[ashley needhamParticipant @ashleyneedham69188]

Neil, you ought to post this nugget on Bobs slo-mo pages!!!

 

Ian, My understanding of trim tabs is that they push the centre of gravity forward a bit, as they effectively lengthen the boat waterline, and thus achieve the effect stated However you need to fit TWO I think, usually they are adjustable to allow for trimming  due to,, whatever..prop handing etc

 

Ashley

Edited By ashley needham on 24/04/2010 19:58:14

[neil hpParticipant @neilhp]

you’re right about needing 2, ashley….. port and starb’d

the best boats to look at and the positioning of them, are the RNLB’s namely  the TRENTS, TAMARS, SEVERNS AND [Ithink] some TYNES AND MERSEYS.

try youtubing for real [not model] lifeboats and watch how they turn in a tight sweep.majic,

and as ashley says.when on the plane, and in lowered trim the tabs both keep the hull  “upright” and the bow from raising  to a state where it “slams” into the wave rather than cutting through it.

neil.

[Ian GardnerParticipant @iangardner62867]

Thanks for the help gents,

I have been looking into this and can’t find anything in my books about trim tabs but notice that one can buy  rather swish looking tabs. They would look better than anything I might cobble together ( I was thinking bent brass or ali) and can be had for about a tenner. I’m not sure what size to get,

or doesn’t it really matter? The boat is 33” by 12” and weighs about 8lb- sorry about old money sizes.

Presumably they are fitted flush with the bottom of the transom? It sounds as though I could expect an improved ‘attitude ‘ from my boat! I noticed it rode flatter with increased power.

 

Ian

[Colin BishopModerator @colinbishop34627]

Here is a lifeboat shot showing how the trim tabs are fitted. As you can see there are three on this one!

 

Colin

 

[Ian GardnerParticipant @iangardner62867]

Thanks Colin,

 

As I thought then- they certainly look purposeful. I’ll be very interested to see what effect fitting a pair will have on my boat. I have also discovered that some model power boats have r/c trim tabs. This was my first foray into  faster boats so I have a lot to learn.

 

Ian

[ashley needhamParticipant @ashleyneedham69188]

Ian, go for big ones.

 

Big ones will either make a visible difference, or muck it up completely, so you can then downsize.

 

i would try glueing on ali tabs with a hot glue gun as a try out, as this sticks well enough but is easy to peel off to try something else or remove altogether.

 

i am sure that trim tabs on destroyers and lifeboats are there to give a clean breakaway rather than shift the c of g like offshore powerboats….I mean…what size tab would you have to fit on a destroyer otherwise?? 

 

I fitted a tab 1 1/2 inches long on the Sunderland, and this makes a noticeable difference to the attitude of the boat at speed..(such that it is)

Ashley

Edited By ashley needham on 25/04/2010 21:31:30

[Ian GardnerParticipant @iangardner62867]

I finally found some info in one of my books- I knew I’d seen something. It was Vic Smeed’s old book Radio Control Model Boats and he suggested prop size squared. I too am a great fan of the glue gun for temporary lash-ups, sticking templates and the like. As you say, it’s easy to peel off afterwards. Maybe I’ll give it a go- quick and easy.

Thanks for the input.

 

Ian

[shipmate60Participant @shipmate60]

Trim Tabs will change the trim.

They change the nose up or down attitude of the hull.

The work by applying force to the transom to usually lift which keeps the bow lower and able to “cut through” the water easier  as well as keeping the driveline at a better angle.

The downside is that they do use power as they cause drag to work.

This is usually counteracted by the more efficient hull angle to the water.

 

Bob 

[Ian GardnerParticipant @iangardner62867]

Thanks Bob,

 

I have now fitted some trim tabs to my Sportsman but haven’t had a chance to try them out. I’m keen to see what difference they make. I will report back when I have done so, just out of courtesy to those who have taken the trouble to help.

 

All the best,

Ian

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